Hi I have a 02 540i and I was wondering if its possible to add a sub and amp to the factory radio? Previous car I had a Bose system and it was a pita because you had to get a converted and all this just.. how hard is it on this car?

It's very doable. How hard depends on your abilities. You should probably use a hi/lo converter. But you may be able to use speaker level outputs if your amp has the proper inputs and you can find the correct outputs.

If what I've said is confusing to you, you should probably have it done by a car audio shop.

Jay

here is a direct plug n play sub kit for your car

Linky (http://www.bavauto.com/shop.asp)

Go to:
Audio/Electronics>>Subwoofer Kits>>Select your vehicle and voila!

HTH