Good info about the 87 300e. Only problem is that it is wrong for an 85 500SEL. Even though the pushbuttons look similar, all the units prior to 86 have separate controllers for blower control and temperature. There are no electronics in the SEL push button controller!
That doesn't mean that the PBC isn't the problem though. It is very common for internal solder contacts to break due to the flexing caused by the pushbuttons and the temp wheel. They are a very good candidate for repairs. Their circuits are easy to see and follow, unlike the later units that do have the controllers internal.
Bosch Master, ASE Master, L1
33 years MB technician