89 S3 3rd brake light bulb and socket?
My 3rd brake light problem was compounded by the fact that one of the little ears that holds the light assembly to the spoiler had broken off. My light was swinging away inside the spoiler. Then I discovered that the whole spoiler assembly is pop riveted together and without disassembling the spoiler assembly, you can't unbolt the light fixture inside!
So I drilled out all the rivets, cleaned out the slot to which the upper-side rivet are attached, slid on some "extruded u nuts" (Au-ve-co part 12669 for the nuts and 12065 for the bolts), creating an upper-rear spoiler which was no longer a sealed "black box."
Then I superglued and epoxied the broken "ear" to the main light-fixture body and also epoxied a "patch" to the top of the light assembly.
Some previous mechanic had taken a too-large bulb (an 1157?) and siliconed it into place. The heat and size of the larger bulb caused the top of the light assembly to start to melt. The epoxy patch (hopefully) will stop that deterioration.
Then I disassembled the original socket which, as mentioned previously in this thread, had lost the little pins that once held the bottom section of the socket to the top half. I reused the existing socket screws (along with two new washers) to hold the new wedge bulb socket in place. Since the new wedge socket fit well into the old opening, I didn't have to slather that junction with silicone adhesive. Instead, I used silicone adhesive in lieu of the broken-off pins to create a new base for the new bulb.
Finally, I ran silicone gasket material both along the spoiler's edge where it meets the trunk lid and also over the tops of the new u-nut bolts (in case the bolt heads are tempted to come into contact with the trunk lid).
It seems to work. Pictures of the assembly steps are attached below.
The miniature wedge bulb I'm using is a Sylvania 194 incandescent. I've got some much brighter LED bulbs on the way and I think I'll end up using those.