I'm new to W123's (bought an '85 300D in Aug.) and it seems to me my speedo/tach instrument cluster lighting is very dim. The rheostat works but even on "full" the lighting is quite dim. I've worked it back and forth probably a hundred times as I read some posts indicating the contacts can get grimy and working the knob can clean them off. As far as I can see there is no back lighting, but rather two (or more?) bulbs in the front of the cluster that shine onto the instruments. Is there any way to get brighter illumination?