If you are into lots of work (removing seats, carpet, drilling holes etc) its a DIY weekend project, gsxr has some pics of a 124 chassis with power wiring:
http://www.w124performance.com/images/W124_stereo/blue_87/
I'm sure the 240D project was far easier, the 124 has power seats (heavy and PITA) and the firewall is way different and finding a thru-hole is not as easy but I think behind the battery is where I would go.
Making a sub box fit the rear deck is another story, when I figure that out I'll post pictures. Too many projects right now!
PS Do you like the Becker tape unit?? (I hate the OE radios!) Make mine Pioneer anytime, Alpine second choice