I was told that the moon / sun roof drain holes could be clogged causing this. But I can't seem to locate the drain holes, etc.
GT500Knights 5/8/2014Technical service bulletin # 77/9
Since august 1981, the area of the spot welded reinforcing plate on the sliding roof fram is being treated with additional sealing material. By doing this, water is prevented from leaking into the interior of the vehicle at the front area of the headliner.
If drops of water appear at the front of the headliner in earlier production vehicles, the front part of the sliding roof frame must be loosened and subsequently sealed along the entire length of the rails (be certain drains are open).
NOTE: water leakage can occur, e.g. When braking the vehicle, because water cannot drain sufficiently fast through the two front drainage openings and then seeps through the spot-welded reinforcing plate on the sliding roof frame.
CREDIT: Alldata Repair
Well, I would take off the headliner, and take out the sun/moon roof and look carefully for the drainage holes. It is saying they're near the front. Look for a spot-welded reinforcing plate and follow it. You will probably find traces of rust/mildew