Good evening
Not necessary to located a S/C for replacement passive belt assembly. 1989 - 1993 MN-12 non S/C are the same. This is what I have learned since I have made many trips to replace the passive belt assembly from the salvage yard.
1. The belt drive normally fails because of lack of lubrication.
2. The motor is the most reliable part of the system.
3. At the salvage yard find 1992 -1993 Thunderbird as I have less troubles with the the assembly. Remove the entire assembly (motor, tract, bracing, mounts, both limit/spool switches) and the passive restraint module. It is located in the truck driver side underneath the triangle cover.
4. Inspect the belt for elongated/damaged holes in the drive track. Reject assembly if found.
5. Once installed lube the track with white lithium grease or silicone lube. Do this once a year for driver and passenger sides.
Good Luck.