1988 Benz 560SL Climate Control

I work for a lady who has a 1988 Mercedes Benz 560SL. She has a problem with the heat and a/c system. The a/c or heat used to just turn itself off after it had been on for a bit, and now she can’t get it to turn on at all, doesn’t matter if she chooses heat or a/c, temp doesn’t matter, just won’t turn on at all. She’s had it in the shop numerous times, and they keep telling her the climate control unit is bad. She purchased a new climate control module through the shop a few months ago, and her shop installed it, but she still has the problem. I’ve taken it back to the shop twice now, and they claim they’ve driven it all over town, and can’t replicate the problem. They now say there must be something else wrong with it, but don’t seem motivated to actually figure out what’s wrong. They’ve told me this is a common problem with Mercedes from the 80s, so maybe someone has a suggestion about what could actually be causing the problem, and what can be done to fix it?