I don't have a wiring diagram for 1800E, but I presume it's similar to my 1800S.

One side of the light circuit has voltage at all times. The various switches apply ground to the other side of the circuit to make the lights come on. Remove the ground, the lights go off.

Ground can come from several sources: 1) the switch under the dash in the ON position, 2) either/both of the door switches when the under-dash switch is in the AUTO position, and 3) the switch built into either lamp. (Now I'm going to have to run out and see if mine lift up -- didn't know that.)

Applying ANY of those grounds should make BOTH lamps light, according to my diagram; even just lifting on lamp would make the other one come on. So the clue is that only the one stays on in your case. That means the wire that's supposed to go from the malfunctioning lamp to the other one has come loose or burnt through its insulation, and is touching ground instead of making a circuit to the other lamp. The problem could be at either lamp, or anywhere in between.

Some disassembly is required...