i dont know what its like with teh e46 but on the e36 the replacement of the stock deck is a quick in and out procedure and definately doesnt need the help of Best Buy. The people there arent really that knowledgable on car audio and you'd be better off buying your HU off a respectable company on EBAY and getting all the new wiring harnesses etc. for half the price and just find a DIY site online.
<img src='http://img268.echo.cx/img268/1430/zkwsig23iq.jpg' border='0' alt='user posted image' />
1996 328is Black
Suspension: MB Motoring 18x8, Kuhmo Supra 225/35/18ZR
Audio:Eclipse CD3414 HU
Lighting: ZKW W/ DDE PLUS