I found this in a 10-year-old thread:
There are two issues to consider with most of our cars... Most of the reproduction taillight housings on the market today are made in Taiwan. While they certainly look just like the originals, they are not on the inside mechanically the same as the Porsche OEM taillights or the German-made reproductions. They have to be modified to remove a divider between the bulbs that is in a different position than original, and then a ground wire soldered to the circuit board of the new LEDs... on the Porsche original taillights, the board grounds to the divider between the bulbs.
Also on the Porsche system, the brake light is toggled on and off for the turn signal function (single filament bulb). Our cars have dual-filament bulbs, one acts as the brake light and the other used for the turn signal or running light. To use the new LED units, you have to wire in a 3-2 wire adapter like you would use for a trailer.