Proceeding with repair attempts... I painted the metal button on the LEDs in the center with clear lacquer to help prevent shorts. Then I tried grinding some "U" notches to try to clear the tin from contacting the center button. I learned that these tin sheets are very soft and easy to bend and mangle. Also, the uniform zig-zag pattern is maintained only by the LEDs, which are originally spot welded or staked to the tin sheets. I'm not sure how durable my soldered joints will be for vibration and cracking in the future. Maybe a few globs of epoxy would help relieve the load on the soldered joints. I see there are some plastic tabs that fit into the zig-zag gaps. The tin soaks up heat like mad, so I had to heat my solder gun as hot as possible, and work quickly. I used a battery to check LED polarity and marked them with red, so I wouldn't solder them on backwards.

 

I got two sets of LEDs replaced, and the whole panel is working now. The color and intensity both seem like a good match for the original LEDs. The only thing I'm not happy with is my solder joints. I have no confidence that they won't crack and come loose. i have an old gun and trigger style soldering iron that I thought put out plenty of heat, but I was having lots of trouble tinning the sheets of copper.
Here's a video showing the two intensities.

Summary:
-I don't think resistor failure is the cause of dead LEDs. All of mine were simply due to dead LEDs.
-Cut the gray plastic case apart roughly where I show in earlier posts. But don't cut close to the red plastic lens, just cut the gray back open. I used a 20,000rpm abrasive wheel.
-Use the same 20K abrasive wheel (or a dremel) to grind off the welded/staked tabs on the old LEDs for removal.
-Order new 625nm 1 watt red LEDs. ($18 for 100 pieces from Amazon - very slow delivery)
-Use a 1.5v battery to determine polarity on the new LEDs and mark them.
-Borrow or buy a HOT 80w soldering iron. You need a big one to heat up the sheets of tin.
-Bend the solder tabs on the new LEDs to make legs that will hold the LED away from the tin. (this will prevent shorting out on the metal disc on the back of each LED)
-For the outer tail-light (not mounted to the trunk) there is a lower bank of 9 LEDs, and three rows of LEDs for the upper and side markers.
(I expect the inner tail lights have similar construction)
-Use a voltmeter to determine polarity of the old LEDs before you grind them off.
-Screw the tin sheets to a scrap of wood before you grind off the old LEDs. Use the existing mounting holes in the tin. This will keep the tin sheets steady and maintain the gaps between sheets of tin as you swap out LEDs.
-On the lower bank, solder in new LEDs in sets of three . Make sure you verify LED polarity before you solder them in place.
-A big 80w soldering iron is needed to solder to the big sheets of tin. They suck away heat from the solder joint.
-Epoxy the gray backing to close the taillight. The cut only affects one mounting stud.

 

Very lazy design on the part of Lexus using a stamped chunk of metal like that. You need a soldering iron like this

I've tried a hot air gun to solder these bead LEDs it works but if you use too much heat solder flows inside the LED and ruins it.

 

Thanks, new soldering iron is on the way.

 

FYI that soldering iron is cheaply made treat it as a delicate instrument. Use anti-seize on the set screw and tip. Not many options for a high wattage soldering iron I had one of these it was even worse
https://www.canadiantire.ca/en/pdp/m...p.0586306.html

 

I've given up. I can't get enough heat for a good solder joint on the large copper sheets. I can't hold the zig-zag gap uniform and even, while soldering the loose LEDs. And I'm not confident I've got the metal tab on the back isolated where it won't short. If somebody wants to try their hand at it, I can send the "test mule" lens and a bag full of red LEDs.

 

Before giving up try a different approach. Solder wires to the LEDs and connect those to the wire lugs. Then glue the LEDs to the board. Another method is use tiny screws.

 

Wow! That iron works! Thanks for the suggestion. I remelted all my cold joints, they look like glass now.

One nice thing I learned. If you do short out any of the LED power connections, nothing smokes. The lights just get dim, or go out temporarily.

 

Here's the struggle for the two upper rows of LEDs. These squares of tin come completely loose when you grind off the old LED. They're normally held by the plastic strip and screws, but while you're soldering you have to find a way to hold the squares stationary, with a parallel gap between them. My fumbly fingers aren't up to the task. Maybe I need to screw them to a block of wood prior to soldering.

 

Yeah, that's definitely a better way to hold the tin.
I think I can avoid problems with the round metal plate on back, by bending the new LED solder tabs down, so the LED stands proud of the tin. The original LEDs were much taller than the new ones. So that looks like a good solution. As you can see, I kept getting the polarity confused.

 

I don't understand this design. The metal doesn't act as a heatsink since the LED don't make direct contact so why do it this way? Should have been a PCB. Don't think you should rely on paint to insulate the casing use a piece of plastic for example from packaging.

 

It works!! Thanks for the tips and direction LeX2K! I replaced a total of 6 LEDs in the bottom bank, and 5 LEDs in the two upper rows. In spite of my horrid soldering and sketchy methods, I'm very happy to prove out the process. I updated the "summary" above on Post #47. The real question is: are Lexus owners tinkerers enough to benefit from this, or does everyone just order new $400 lenses?

 

Can get them for about $280 per side most people are going to do that instead of cutting and soldering. How are you planning on sealing the housings?

 

The metal does act as a heatsink. The tabs on the LED connect to the semiconductor Diode (anode & cathode) and carry heat away from it.
In VERY high powered LEDs they connect the semiconductor substrate to a separate heatsink but they don't normally bother until the LED is above 5W as heat dissipation via the anode & cathode is enough.
PCBs can act as heatsinks but if they get too hot, the copper separates from the board and can break the circuit.

This method of manufacture is a PITA for home repair, but easy/cheap for manufacturing and for reliability.
If the LEDs hadn't crapped out his 17+ year old taillight would still be working 100% so the circuit assembly method was good.

 

This particular taillight is going in the bin when I finish tinkering. I have another with only two bad LEDs that I think I'll repair and sell for cheap. I'll run some sheetmetal screws into the plastic slot I cut, for stability. Then run a bead of silicone sealer to close the rest of the gap.