The fans run in parallel so whatever triggers the left fan triggers the right fan simultaneously. Either the left fan is stuck on in isolation, or a sensor is stuck and the right fan is dead.

If there's a 3-pole temp switch on the turret fitting for the upper radiator hose, remove the connectors and see if the fan turns off. If so, the temp switch and the right fan are bad. Or your engine is overheating

If there's a temp switch with connectors on pigtails on the AC receiver/dryer, remove the connectors and see if the fan turns off. This temp switch taps into the refrigerant circuit so don't remove it without first discharging the AC system.

Those are the fan triggers on an 86-87 SDL. The 90-91 SD/SDL might be different.

Sixto
87 300D^2