This FAQ contains information about the bad taillight ground on first generation four door 900.
Maintained by Thor Carlson (firstname.lastname@example.org)
Directional lights don't work properly
Green signal indicators in the instrument cluster glow when the brakes are applied
All directional lights and flashers glow when the brakes are applied
This behavior is common on the four door first generation 900's and is caused by a bad ground circuit at the driver's side taillight assembly.
On the four door, squeeze the plastic clips to remove the driver's side taillight assembly from inside the trunk and note where the heavy black wire is grounded to the printed circuit. The ground circuit itself may well be blackened and the adjacent plastic partially melted. Fix the problem by soldering or screwing a supplementary ground wire from the ground tang on the light assembly to chassis ground (there's a good spot on the frame member under the trunk floor). It's a 15 minute fix!