I own a '97 M3 and was thinking of doing the same thing as the original poster (but possibly re-locating the stock unit down low for easier Empeg removal). I've been using an Empeg with my home stereo for a few weeks now and absolutely love it, especially compared to the nearly useless Rio (nee, Dell) Receiver under it... but I don't want to give up the option of playing cassettes and CDs if I do not absolutely have to.

Has anyone tried this? Can anyone state for certain that there is (or is not) enough room behind the sunglass holder or OBC?

Two possibilities come to mind for inputs, since going straight to the amp is a poor option. The simplest would be to use an RF adapter (ala after-market CD changers). The other would be to use the CD changer inputs, but that probably involves more hackery than is worthwhile -- especially if trying to retain use of an existing changer.