I've been having some trouble trying to figure this out. On the dashboard, I have a PRND2L display of which mode/gear the shifter is in on my automatic transmission. All lights except 'D' are working.
So, must be the light bulb, let's replace it. Nope... still doesn't light up.
Swap it with a known working one? Nope... Now the old D bulb lights, and the working one now in D doesn't.
So, let's look in the wiring diagram for 1991 to see where the connections are... Nope, the wiring for the dash doesn't show the PRNDL...
Since it's not the bulb, it's probably the switch on the side of the transmission (they become un-sealed and corrode internally) or the wiring to it (look for rubbed through wires where it goes over the transmission and around/over the front fuel tank).
This is the clearest wiring diagram I've found.
It's curious that Mitsubishi added all this wiring just to put some pretty lights on the dashboard, but couldn't be bothered making a proper Power On Self Test circuit for the battery/AT/water separator warning lights.
