Originally Posted by SD Blue

Mostly, it is just resoldering a problem area.

The circuit boards are arranged in an "H" pattern with solder connections along the edges where they meet. Over time, with flexing, temperature changes and just general use, the solder joints develop cracks and need to be resoldered.

Here are some good pics (post 36 &37)
http://www.peachparts.com/shopforum/diesel-discussion/86958-diy-page-introduction-resoldering-climate-control-unit-3.html

And some schematics:
Mercedes-Benz 126 Service Manual Library