I have a 91 300SEL and the blower motor does not work. Since I have a 91 560sel as well, I started to swap parts one by one to rule them out.

Checked all fuses which are good.

swap Push button unit from 300sel to 560sel and it is good & vice versa

Swapped both blower and "porcupine" from the 560SEL (known good working unit) to the 300SEL and still does not work.

The only thing left is the resistor in the engine bay but how in the world do you get to it. I tried removing the fuse box but looks like I will also need to set aside the bulk head as well