The 2006 Jetta has a LED array that illuminates the 3rd brake light. In my case all the LEDs were not activated when the the brakes were applied. The two tail lights worked fine, the LED array stayed dark.
Of course the prevailing knowledge is that there are no serviceable components in this light assembly. A replacement part runs about $50 online.
I decided to take a look and see if anything was obvious...
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Step 1: Step 1: Remove the LED Assembly From the Car.
First the assembly must be removed. Simply wiggle your fingers under the cover and pull. there are about half a dozen clips that hold the cover on. You have to pull fairly hard, once you get a couple of clips you can see easily where the other clips are.
The assembly has a two wire connector that will come out when the keeper latch is spread.
The assembly is attached to the rear window with a clip on each end. To remove it, push the assembly towards the back of the car. Some force is required.
Test the two wire connection to ensure that, when the brake pedal is depressed there should be about 12 volts present.
At this point, if you have purchased a new assembly, just reverse these steps and you are finished.
Step 2: Step 2: Disassemble the Light.
The light is made up of a LED array, magnifier, clear diffuser and a red reflector/diffuser.
There are two tabs on the back of the assembly. Start with the tab opposite the electrical connector and with a flathead screw driver, lift the tab and push the lenses. You will need to gently work both tabs a little at a time. When both tabs are disengaged push the end opposite the connections out first.
As the lenses come out you will see that, with the exception of the magnifier are really not connected to one another.
Everything is held together by the two tabs.
Step 3: Step 3: the Nitty Gritty.
There were no obvious areas of cracking or burned/discolored areas on my LED array.
When I plugged the array into a 12v supply I noticed that the LEDs all came on but extremely faint.
My power supply was actually outputting ~ 14v the car was showing about 11v.
Next I tested the surface mount components for continuity and resistance. I noticed no anomalies. I did have to scratch some of the surface connectors with my test leads to test for continuity.
When I reapplied the power the lights all came on bright. They were still not working when connected to the 11v in the car. I surmised that one or more of the connectors was adding resistance.
I re-soldered all the surface connections. Or more exactly, I heated each connection to re-flow the solder. Since all the LEDs were working, or not working at the same time. I assumed that their connections were Ok and I left them alone.
This did the trick. Even 11v in the car brightly lit the LEDs!
Step 4: Step 4: Reassembly.
To reassemble, stack lenses and LED array into the red reflector and slide into the black housing.
Sliding the end with connector first so that the two leads will protrude out the back.
The two tabs will snap back into place when the lens are seated.
Reverse the previous steps.