I installed a sub, with a separate and additional amp, in the trunk of my car - an Alpine 350 watt sub / amp package.

here are some points raised above....

1) The subs under the seats are really just very good mid range / woofers. Adding a sub will really beef up the bass.

2) Always take sub amp power directly from the battery, for two reasons. Firstly, and foremostly, the power requirement of a sub amp is huge, so you don't want to be overheating any wiring that isn't man enough for the job and having to pester the insurance company after a fire! Secondly, if you take directly from the battery, you can be assured of a "clean" supply, so no buzzing, popping etc...

3) In the trunk, there is a accessory socket - use this as the remote switch for the amp - to turn it on and off - much easier than running a wire from the head unit.

4) E60s are fantastically sound proofed. You need to think about how to get the sound from the trunk into the car - I have a ski hatch, and had a grill custom made to replace the ski bag cassette - exactly the same colour as the upholstery, looks and sounds great.

5) It's really easy to get your speaker level inputs from the speakers in the rear parcel shelf. Just prise the grills off, remove the three screws holding the speaker down, and using somthing like a knitting needle, push your new wire down through the sound proof foam into the trunk.

6) If you are going to secure your sub to the trunk floor, for goodness sake check under all the carpeting / sound proofing to see where your screws are gonna end up - there is a huge wiring loom under there, and the cable that take the battery to the engine compartment.

7) To make the bass sound really crisp, install capacitors in line in your existing speakers. This will omit the really low bass notes from those speakers, meaning that the won't distort, leaving your sub to do the hard work.

Think that's about it....if I think of anything else, I'll post again.

Rob