You guys are life-savers!
I too connected an extension wiring harness to the connector near the driver's side rear wheel, and it was dead.
I can confirm that installing Lexus part 81985WY010 (I paid $74) rectified the problem and once installed, the connector was live.
You should also make sure that you have the 30A fuse installed in the trailer fuse position.
In case you're curious (I was), the 81985WY010 contains a circuit board with 3 FET's and what could be a micro-controller. So, at least your money isn't just buying just a simple jumper that's making Lexus a ton of money. They're still making quite a bit tho, those parts are probably worth $20, if that...
Good luck to those DIY's! (BTW, my dealer wanted to charge me $284 (2 hours labor) to install the part. I did it in 1:15.)

 

I realize that this is an old thread, and also that this is my first post here. I just wanted to reinforce the fix above. My 2011 RX350 had the 4 pin trailer plug behind the driver's rear tire and the 30 amp fuse in place for the trailer lights, but no voltage. I purchased the 81985WY010 from the Lexus dealer ($65), snapped it in and got lights. I also did not remove all the stuff in the instructions, just the vertical plastic cover on the passenger lower right, the bolt behind the tie down cover, the bolt behind the net retainer piece, and the small black tray on the right side of floor space, pulled out the panel and connected the converter. Maybe took 20 minutes. Thanks to all who posted earlier, I would never have figured it out.

 

I have 2010 RX 350 with Tow Package, standard 4 pin wiring, and installed a 5 pin adapter except for the backup wire connection. Has anyone done this for a 3rd gen RX350, and if so where did you get the backup wire connection? I believe it could be tapped from the rear hatch (not easy), in the drivers kick panel, or at the shifter console. For the 3rd gen I believe it is not accessible in the trunk drivers side, as I’ve removed the trim there and can’t find it with a sensor. All the tail lights, yes, but not the reverse light in the hatch. Here is a link to Askanexpert where they try to answer this problem using the Lexus wiring diagram. Maybe running a wire from the front to the trunk could be easier than removing the hatch trim and snaking the wire somehow to the trunk. https://www.justanswer.com/lexus/70g...ht-circui.html

 

Curt trailer hitches and etrailer both have clear YouTube videos that show how to install a 4 pin trailer harness to a Tow Package ready RX 350, and a different 4 pin harness requiring minor trim removal if you don’t have the Tow Package. None of them address how to connect a 5 pin harness in an RX 350 3rd gen, which requires a backup wire connection to control surge brakes on modern trailers. I have a recent post in this forum asking for any information on how to connect to the backup wire in a 3rd gen RX, “Trailer Surge Brakes Require Backup Wire”. Does anyone here have experience with this?

 

I got no suggestions from my post on this. Guess not many of us have surge brakes on their trailers, or need to locate a backup wire for a camera or rear fog light.

Anyway, today I solved the problem. Open the middle wire loom rubber conduit that connects the hatch to the headliner. Locate the backup wire in the loom using a tester, with the car in reverse but not running. Mine was light green. Pull down the back of the headliner. Splice a wire to the backup wire, run it to the left side of the headliner, and fish it down the left inside channel of the trunk. Remove the trim on the left floor of the trunk, and the trim on the left edge of the trunk. Finally, attach your new backup wire to the blue backup wire from your 5 pin to 4 pin trailer harness adapter. This splice stays out of sight under the left cubby tray in the trunk. The job took about 45 minutes. See the 3 photos below.

[img]blob:https://www.clublexus.com/8a7e5f7f-c88b-434c-ba5c-45d6c58d1ba1[/img][img]blob:https://www.clublexus.com/05dec566-2f4b-4bcf-8927-af14e774fc33[/img][img]blob:https://www.clublexus.com/f01fea8a-f2a6-4eba-af08-d73adb091dbf[/img]

 

Thank you GPOM for your information. I have a 2013 RX350 with tow package and I have now hooked up the seven-prong trailer connector. I have everything hooked up except the backup lights. I will start looking for this wire to hook up to the wire on my seven prong for my backup lights on my trailer.

 

For those that have the tow package on 3rd generation RX and want to install the 7-prong connector for brakes, power to the trailer, and backup lights. Do not use the E-trailer method of drilling a hole through the fire wall. There is a rubber gromet on the passenger's side under the glove box that you can run the wires for the brake controller through.