I have to add a big thank you for this post. I know it's a few months old but in searching I ran across it in replacing the antenna on my '83 W126. There was a Kenwood cassette deck that was installed ok - thankfully whoever did the install just tapped the harness. Replaced it with a JVC CD player. There was a constant 12vdc at the antenna plug in the trunk (circuit shared with trunk light) but no 12vdc signal. When I pulled the radio harness out the was a single black lead with a bare spade. The wiring diagram shows a blue/red striped wire instead of a solid black from the radio to the antenna switch. Using the switched lead from the radio to the black lead solved my problem.

On a side note I replaced the bulbs for the center console switches/shift quadrant and HVAC. Using fiber optics is a nice idea but the light spider is not an easy to put the socket back in (at least when you have big hands).