This is one of the first mods I did to my car, and probably the most useful. A power trunk was something I just couldn't do without. I figured out how to get to a hidden channel in the keyless entry module that can be used to pop the trunk. I can pop the trunk from in the car and from the remote. I used a BMW E36 power trunk actuator. I made a new circuit in the fuse box to accommodate the power trunk and other things. It also powers my power fuel door and floor lights. There are a few open spots in the fuse box that aren't used. You just need a good wiring diagram (and a general understanding of wiring diagrams) and you can isolate your wiring from the factory wiring. You can see the 4 wires I added going to the new fuse in the pic. Your car will not have these wires there in your fuse box.
You may have noticed the extra button on your remote. It actually does do something. If you take it apart you will find a small piece of plastic that is preventing the button from moving up and down. You have to remove this small piece of plastic. When you press this button on the remote it sends a pulse to the hidden channel in the keyless entry module. I used this hidden channel to pop the trunk.
I debated where the best place for the switch would be. I originally modified a Mazda switch and put it with the other switches. I didn't have this setup for very long before I came up with using an Audi power trunk switch in the center console. I ran the wiring from the fuse box to the power trunk switch and from the switch to the actuator. This is to pop the trunk from inside the car. I started off with just one switch in the middle, but I later expanded it to three switches to do other things.
To pop the trunk with the remote, I ran the wiring from the keyless entry module to a SPDT relay to the actuator. I ran all my wiring to the trunk through the factory loom. I extended the loom at the second license plate light. The wiring ends here with a Mazda plug. This goes to a Mazda to BMW harness I made for the BMW E36 actuator. I mounted the actuator and cut and bent a rod to the right size to attach to a small clip that is in the trunk latch.
Original installation May 2009