Gilly, this problem is a challenge. I appreciate you taking the time to work through it. I pulled the cluster out and looked for the small circuit board you referenced, but could not find it. I took three pictures of the back of the cluster which I will embed below:
I have been also trying to figure out the three other related (I think) problems; turn signals & hazard lights not working, power antenna not working, and release of the hard top just flashing the swithch light. These problems along with the instrument cluster guages not working (excepting coolant temp) seemed to occur at the same time when I got the car out of heated winter storage.
If your interested the links to see the car follow:
http://www.concentric.net/~Whitcomw/images/MB300SL/General/PIC00001.JPG
http://www.concentric.net/~Whitcomw/images/MB300SL/General/PIC00002.JPG
http://www.concentric.net/~Whitcomw/images/MB300SL/General/PIC00003.JPG
Again... thanks for the help.