I had a problem similar to yours. Turned out the circuit board which houses the speedometer, temp. gauge, fuel gauge, and all the instrument lights, was burnt out. I had to dismantle the cover over the steering wheel (two screws), take out the knobs for the lights, take off both right and left covers to the panel, unscrew the two screws on both sides and slide the instrument cluster out, to the point where I was able to disengage the speedometer cable from the rear. Slid it out a little further, and disingaged the wiring. The instrument panel was able to slip out to the right of the steering wheel. The three resistors were burnt. Repaced it with a used panel, but I was able to take off my old speedometer and put it on the replacement fairly easily (no springs to worry about, but there were a few bushings behind the fuel gauge and temp. gauge which can get lost easily). My alternator light now works, and it's charging! Good luck!
'84 240DL.