Audio options?

Hi, Just bought a used 2000 528i. I'm curious as to 3rd party audio upgrades. My 528i has a factory radio/cassette deck. There is a CD/T message on the radio, and a place for a deck in the trunk. I don't have the owner's manual (it's on order) so I don't know how it would work - prewired maybe? I see crutchfield has no solutions - so I'm getting worried. Any suggestions? I'm looking for a CD player - and would love to add XM sometime.
Thanks Bruce.

The cd players are prewired with an i/o port. I just bought one on ebay for my 2000 528. I got the BMW model, although pioneer and alpine probably also work. I know that there is a bracket required, so make sure to get that at the same time.
I logged on because now that I have all of this stuff, I'm trying to figure out how to mount it - I also have a telephone kit installed in the same area and its crowed in there. If you learn how to do that part let me know.