Hi, I just did this yesterday and can share details on how I did it and pics if i get around to upload them. I just sold the RX though so I prob will not be contributing again. Since I gained info from you all I thought I'd contribute back

Summary: No need to remove bumper entirely but disengaging the side portion helps a lot. My light assy came out easily but the plug was super tight. No need to jack up car. I purchased a new TYR brand rear marker light assy w/ bulb from Rockauto.com for $53 shipped. It appeared every bit as well made as the original Toyota part. In fact, the new part has a better rubber plug on the back to allow evap of any collected water inside the lens. Otherwise fit was a smidge looser until reassembled then it was same as stock. Allow an hour or so.

Tools needed:
- 10mm socket (1/4" drive preferred but 3/8 prob works too)
- Flexible shaft socket driver OR wobble head U-joint to match socket above
- 14mm socket
- Socket wrench
- Phillips screwdriver
- small flat head screw driver
- needle nose pliers (I needed em, you may not)
- small jewelers screwdriver or pick
- safety glasses if you choose to crawl under car and don't like crud in your eyes

Steps:
1. Apply parking brake

2. Just underneath rear corner of car, behind rear wheel there is a black plastic cover approximately 10" x 16" held on with a 14mm hexbolt, a 10mm hex head screw and a Philips head screw. Remove these fasteners. Push UP (not down) to disengage a plastic press fit fastener along edge of bumper cover then twist/tilt/maneuver this black cover out of the way. Why Toyota uses 4 different fasteners here IDK.

3. Inside rear fender well, locate rearmost portion of fender liner. Pull forward from bottom edge to release plastic press in fasteners in middle of fender liner. Mine really popped/ flew. I used a small bungie attached to the strut/spring to hold it against the tire and out of the way.

4. Looking underneath the bumper at it's topmost point closest to rear wheel is another hex bolt, either 8mm or 10 mm. Unscrew.

5. Gently pull painted plastic bumper cover (NOT the black plastic inner bumper support) away from the car. There are 3-4 tabs along the top edge of the bumper cover that need to be released from their slots. Be gentle, wiggle and you should be ok.

6. Find the single 10mm bolt holding the rear marker lens in place. With the side of the bumper released it allows you a lot more room to maneuver your hand and socket. I used a 1/4" flexible shaft socket driver cuz its what I had. A wobble head ujoint works here too. Unbolt the lamp.

7. Disengage plastic pop in fastener holding elec wire for marker light.

8. Use small flat head screw driver to disengage plastic pop in fastener just under the lower, rearmost edge of bumper.

9. Holding the bumper cover away from car, pivot out marker light to inside the bumper cover. I found it helpful to lay parallel to and underneath the rear bumper so I could have one hand on the outside and one inside so as to hold/guide the marker light in and down.

10. Marker light should be hanging down below the bumper by the wire. Unplug wire at bulb or twist and remove entire bulb socket, then unplug. I used a needle nose pliers here it was so snug. Your entire marker light should be removed now.

11. Now is a good time to inspect and clean the female side of the plug. Mine was a bit wet/corroded so I cleaned up the contacts with a jewelers screwdriver.

12. Remove new bulb/socket assy from new marker light by twisting 1/4 turn. Plug it in and test by turning on headlights. The contacts on my new bulb were buggered so I straightened them and voila, it worked.

13. Assemble new bulb/socket into new marker light.

14. Reverse above steps to reassemble.

15. Reward thy brilliant handiwork with a sparkling malt beverage of your choice.

 

Do it yourself! 15 min max! Online sidemarker cost me $45 included new bulb. Used 1/4" ratchet with 10mm socket and 14" extension to get to the ONE bolt securing the marker. The bolt is actually at about a 45 degree angle from the light, so angle the extension toward the wheelwell. With bolt removed (no removal of bumper screws/bolts, nor jacking up of vehicle required!)..sit down behind the bumper to remove the marker by reaching underneath and from the outside. (Don't try to disconnect the light plug until after the marker had been almost removed). Here the jiggling..and maybe some frustration starts!!! The marker has a lip at the top and on the non-bolt side....you'll need to work the marker to free these, then move the "bolt side" of marker down and out first. Disconnect the light socket plug, then pull the marker out. Installation is reverse...but make sure the light plug is attached before the marker is worked back into place.

 

Thanks to this thread! Changed my right-side marker today, old one was broken. Nothing was unscrewed from the bumper other than the single bolt holding the marker.
To reach the bolt, I used 1/4 10mm socket + 1/4 u-joint + 1/4to3/4 converter + 3/4 10'' extension + 3/4 wrench. I didn't have 1/4 wrench and long extension, so used 3/4. But 3/4 u-joint was too big to reach the bolt, thus converted it to 1/4.

Couldn't bet 15 mins for replacement it took me ~30 mins overall, where 20 mins were spent to figure out how to remove the old marker from the bumper after unscrewing the bolt. Need to play with it, the space is really tight but possible to remove. The marker went into the bumper, then rotated less than 90 degrees bolt side down, and was pushed down. The new marker went in very quickly by the same path reversed.
Of course, a mandatory part of the job was to test the new marker's light while it was hanging on connector, before installing it permanently

 

I thought I'd contribute to this thread as I couldn't find any pictures on how to replace a rear bumper side marker for the RX330. This should also apply to the RX350 as well 07 to 09. I partially removed the bumper to gain more space to remove the side marker. Others have done it without detaching the bumper, but it is a lot easier this way and not much more effort. These are the tools used. 10mm socket, u joint, extension, and flat head screwdriver, and Phillips screwdriver not pictured.

 

Remove 10 mm bolt At the bottom corner of the bumper

 

Rmove 10mm bolt from inner lining (not pictured) and pry out the top retaining clip from the edge of the bumper. It's will break and will need to be replaced. Do not remove the lower plastic clip. It is not necessary to remove the fender well. It will pry open and you will have enough space to remove the side marker lamp. Not pictured but there is one more bolt you will need to remove. First you need to pull out the top edge of the fender well to reach it. Feel for it as it will be obstructed from view. It is the upper bolt holding the bumper to frame located on the top right edge behind the fenderwell.

 

Open the trunk and remove Phillips screw and retaking clip

 

Remove plastic retaining clip with flathead screwdriver. Wedge it in between the plastic top and bottom then pull it out so it won't break.

 

Pry open the rear bumper

 

Now you have more space to reach the side marker from underneath. Remove the 10mm bolt using the socket wrench, extension and u joint from underneath. The lamp is now dangling. Unplug the lamp and remove.

 

See the new and old side markers. I got the TYC aftermarket lamp from EBay for around $50. The TYC is an exact match and perfect fit. I had used red reflector tape over the old lamp but the Motörhead in me says to replace it.

 

Reinstall everything the way you took it out. See the finished product and enjoy a cold one.

 

Here's another picture of the finished product

 

I replaced mine without taking the bumper off also. Hardest part was turning the one screw that holds the light, even with a u-joint socket, I could only turn it about a quarter turn each time and was hard to get my hand in there. Not a big deal though and it did come out with only a minute or two of wrangling. I sprayed the screw with WD40 before putting it back which helped installation.

 

Thanks for posting everyone. Using the knowledge gained from the forum, I saved $170. Local shop quoted $240 to replace the marker light; $100 for the light and $140 for labor. After reading the posts here, I ordered the marker light online ($56 plus next day shipping & tax; $70).
The next day when the light arrived, I replaced it in 15 minutes once the car was in my garage. Easy - really easy - to do and I didn't remove the bumper. I removed 3-4 screws from the bumper as described herein, pulled the bumper back a bit, and had plenty of access to get to the screw on the marker light with the U-joint socket.
Again, thanks for saving me a bundle. Knowledge is power and more $$$ for the early retirement account!