trent, it is good to hear that you can use "non-free air speced" speakers in free air. i totally agree with your logic (space saving, ease of installation, ect.)i used free air 12s (pyles) in the rear deck of a '79 trans am. the package tray was cut out , .75" MDF was placed in there, and they sounded great. did you do the install on your CLK? i assume that you rewired all of the speakers. the factory wiring diagram shows "speaker division points", which i have no idea where they are. this is important if you are going to use the factory wiring because of the way they wired them (parallel, series, combo..). i have measured the resistance in the trunk (speaker outputs from the factory amp) and have come up with some very strange readings. this makes sense when you discover that the fact speakers have very odd ohm ratings. my point is that if you are going to upgrade the speakers, they will need to be rewired. has that been your experience? without running 4 channels off the OEM HU, the fader will not work for sure, as you know. it may not be possible to get fader function from the OEM deck with an aftermarket amp at all. i dont know. having the battery in the trunk does make things alot easier.

questions:

what did you put in the triangles? how are they wired?
does your car have factory phone and or steering wheel controls? if so, did you have a problem with the phone??
do you know where the "speaker division points" are located??

thanks for sharing your experience. when (if) i get my new CLK, i am going to try to find a way to use an after market amp, use custom plugs, consider free air vs an 8" sealed (very small enclosure), and keep OEM HU. the amp will be the "entry level" MTX 4 channel. the trick will be good, cheap components that far out perform the OEM system without splicing into OEM wiring. ill keep you posted if you are interested..