1984 Volvo 245 DL overdrive dash light not working
 

per the wiring diagram, the light is connected to the same power output that drives the OD solenoid, and the light is grounded in the instrument panel (via the same ground as the turn signals, the high beam indicator, etc). its a yellow wire that goes from the OD relay pin 87 to instrument panel connector 31 pin 4 (31 is the full circle connector on the back of the instrument panel).

note, the instrument panel has TWO DIFFERENT OD lights. the MANUAL transmission uses the OD light just left of the trip meter reset button. an automatic OD, has an "OD Off" light, often a yellow up arrow, thats tto the far right of the indicator lights, next to the bulb failure light.