Just replaced the head unit in my 78SC from the original cassette. Thought I would share a few highlights.

The original rear speakers were not stereo, were wired together so had to run new speaker wires.
All the factory speakers were 8 ohm so had to replace with 4ohm.
Only Blaupunkt overdrives 5*7 would fit in the rear on the existing studs on the package tray. The speakers had "hidden holes" in the metal of the speaker and I had to file away the rubber seal to get to them , be careful not to pierce the speaker.
the door speakers I also used overdrives 5 1/4 but had to modify the plastic mounting cover that they mount to on the door to get them to fit.
For my head unit I found a spare yellow connector for illumination ( 12v with lights on ) under the dash.
The factory radio used switched power and i took my constant from the fuse box, the second from the windsheild I think and it wasn't being used ( ac blower ) and I don't have ac.

The antenna reached the new head after I took out the box under the floor boards passanger side.
The antenna power signal worked with the new radio no problem.
The flip face actually clears the lighter and the other *****.
Great afternoon project with rewarding reults.