Originally Posted by CanadianX5

If your adding a sub amp that puts out more than 100 watts you cannot tap into the 12v switched output in the rear or any other wire.

Bmw uses wires that are dedicated for there purpose meaning that if you tap on an addition current draw like an amplifier you are definately going to be blowing fuses or worse.

If the power draw is very close to the fuse its tapped into,the wire could heat up and could eventually cause a fire.

Always get the power for an aftermarket amp at the battery or the main battery distribution box. In our X5's its a little difficult to connect to the battery terminal because there is a special connector on it.
I would suggest getting power from the distribution block which is located at the back left in the spare tire well.
It's really easy to find and to connect to.
Also use a minimum of 8 guage wire for the amp you described.

As for running wires from the front-------
Take your time, try not to get frustrated and break something.
Most interior parts will pop off but some have screws but you can usually find them.

If your willing to tackle this have fun and good luck.
If you feel it a little overwhelming get a quote from you local stereo shop.