On my '87 the cable broke off and separated from a plastic piece that bolts to the roof glass. Without the cable(s) being attached and working the glass could not maintain pressure on the rubber seal. I used glue to repair amazingly it is still fine 5 years later, the type of plastic melts in contact with superglue so it basically made the plastic one piece again.
You might have a different problem but the type of failure I had I think is most common, that and the motor itself failing. Removing the motor assembly will confirm the problem you will be able to see both cables they should move with the roof. Pulling the motor is a bit tricky without damaging the headliner.
Best image I could find here's the cable that failed on me, I circled where the cable broke off from the plastic. 63221 is the housing the cables ride in, sometimes the cables seize in the housing.