I doubt you're having the same problem I had, but what the heck?
The a/c in my 560SEL wouldn't come on, either, until the ignition key was wiggled in the ignition switch. There is some slight play (that I'll probably have to repair one day) in the switch and if the key was not fully turned, the a/c wouldn't come on -- no fan, no nothing. Dealer showed me what was wrong and said it's a common problem. Switch can be just slightly off position and the engine will continue to run but the a/c won't go on. I would have lost money on that bet.
1989 190E 2.6