Here are the steps I used for adding a 2nd trunk light to my 1988 325ic:

The whole process only took about 15 minutes.

1. Open the trunk.

Previous single trunk light

2. Remove current light from trunk using small flat bladed screwdriver. There are two wires connected. A Purple-Brown (PRP-BRN) ground wire and a Red-Green (RD-GRN) live wire.

3. You will have to create a jumper, or splice in the third light. I chose to make a jumper with leftover BMW wire (I got the right colors). Basically, if you haven't created a jumper before, I made it with one spade connected to 2 connectors. This way, the two lights are in parallel, not series. That way, if one light goes out, the other is still lit.

I used roughly 18 inches of wire for the long strand and about 2 inches for the short strand.

A picture of the jumper after connected. Notice the correct color and the 2nd wires running to the other light. Spades are blue, connectors are red.

4. Run the two longer leads of the jumper back into the original mounting hole, through the trunk housing and out the square hole on the opposite side of the trunk latch.

Lights connected after running through trunk lid.

5. Connect all the new connections to your lights and mount lights into trunk lid.

Picture of 2 trunk lights

Enjoy!