Do you have a US car? The CD manual covers US versions of the W126. You talk about AC knobs. All US W126 cars came with ACC which has buttons, not knobs. The manual will help you with most procedures, but I'm pretty sure it doesn't cover non ACC systems.
As for the lighter, look for a cluster of 4 wires. Two go to the lighter, two go to the little bulb in the ashtray. I'll look up the colors when I get home.
You might check the Haynes W124 manual available in English in the UK. It might have AC info that you can use. I have the book and I'll see if it has anything on the problems you describe. Another source is the W116 manual. I know the paper copy has both AC and ACC. I also have that volume but I find it difficult to understand.
More than a bad switch, I suspect that whoever did the hacking in the console disturbed the vacuum hoses.