99 SLK 230 ?'s

Most of the Bose systems have individual amps on each speaker. You might run into problems replacing the bose head-unit because the bose speakers sometimes have an impedance of 2 ohms or lower, and an aftermarket deck can't handle that load. If you get a new head-unit you will at least have to run new speaker wire (I would replace the crappy bose speakers anyway).

anyone else?Best Buy Installer

you can get away with triggering on the factory amp and just feed line level outputs to the factory harness, which should be the same as a VW harness. the biggest drawback to this is that even at minimal volume using the aftermarket h/u, it's still louder than one would expect, but many people don't mind. and if you're installing a h/u that beeps with each press of a button, you may wish to see if you can mute it, because it could get pretty annoyingly loud.....especially if it's a sony h/u. there's really no need to worry about the air bag in the door. i've never removed a SLK door panel, but i have done a 99 C230 sedan door panel, and there shouldn't be very many differences with many german vehicles, including the C230, you can pry out that little sign that says "air bag", which will probably reveal a screw behind it. you also need to carefully pry out the plastic piece behind the door latch handle (that thing that you pull in order to open the door). also, the grab handle will probably have a cover that you need to pry off which will reveal another 2 screws probably. then just pry off the door panel since the only other things holding it in place should be just the regular clips. if you have a tough time with the window edge portion, don't try to lift too hard on it. instead, try pulling towards yourself. many german door panels come off that way more easily than lifting up. audi is the only german i can think of that requires you to lift up instead of pulling out, because they use hooks to hold the door panel (like a ford thunderbird). oh, btw, you will probably need to unscrew the lock pin first. good luck!