Three dash lights
 

On my 1978 300CD, when idling, the Battery and both Brake lights on the dash come on. The faster the RPMs, the less the lights glow, becoming extinguished when the running at about at about medium speed. I had Autozone check the alternator, which they say is good. Any ideas??

Thanks,

Bud

I'd say one of the diodes is going bad.

I've never worked on that vintage MB, but I think you can buy them separately of the alternator and change them yourself.

Brushes on the voltage regulator, is my stab at it.
(held in by 2 screw on the back of the alternator - I'n guessing the brushes might be little nubs)
That, or maybe a bad ground...but I'd check the brushes first.