Next time post something like this in audio. I can't help you with exact instructions on trim removal, but the DIY in my sig has helped people with other models and the basic principals are the same.
First, you need to install an RCA converter. If you ever plan on replacing the HU, do it back there. If not, do it at the factory amp in the trunk (or off the rear speakers if the amp isn't in the trunk). Splice the wires onto the corrosponding wire (rear left + and -, rear right + and -) and you can run the RCA.
Next is the turn on lead. If you don't want to run a wire to the HU, tap any switched 12V source. You can also tap a constant source but you'll need to install a switch.
The next step is to wire the power and ground. Connect one end of your power wire to the +bat term and the other the +term on the cap. Use rin connectors on both. Next, run a ground from the cap to any exposed bolt. If there are none, drill a hole with a sheet metal screw and secure it with a ring. Charge the cap as per manufactures instructions.
Now run another power wire from the +term on the cap to the amp, and a ground wire to the same spot as the cap.
Also, make sure to install a fuse on the battery.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>