my 1991 w250 with 5.9 ltr cummins turbo diesel has a problem, when its in park the volts reads 12 and the head lights are bright put it in gear volts go around 8 head lights almost ofradio flashing all the dash is blinking put it in park again and back to 12 volts

 

If its cold that could be the grid heater cycling. The volts will drop and go back up a couple times while the truck warms up. Its enough to dim the headlights.

But if its only doing it when you put it in drive, idk. Could be short.

 

Pull the voltage regulator off the firewall and make sure it has a good clean ground.

Dave