Well, several years after the original post, but I thought I would relay my experience. Hopefully this will be helpful to others.
I have a (new to me) 95 T. It had a horrible squeak/rattle/vibration noise coming from the general dash area. It kind of sounded like it was coming from behind the center vent, or maybe the glove box. First I though it might be a squeak from the rubber at the T-top like others have reported. I bought some Shin-Etsu grease online, and applied it on all my window and top gaskets - no real change. So, with some trial and error, and with a passenger helping me, I found, like ibDursty, that if a few pounds of forward pressure was applied to the dash just above the glove box, the noise went away or was greatly diminished.
Thinking there must be something rubbing behind the glovebox (SRS unit or a duct maybe), I resolved to remove the glove box so see what was going on. I couldn't find anything. But, while working on putting the glove box back, I found that I could replicate the noise by pushing forward on the dash just above the glove box while the car was standing still.
With an easy ability to make it squeak when I wanted it to, I determined that the noise definitely was coming from the joint between the dash and the defroster vent plate at the front of the dash (see pictures).
Using a pipe cleaner by folding in half and then twisting it together, I applied the Shin-Etsu grease all along this joint. It seems the dash and vent plate clip together at three points along the joint. It's where the joint gets tighter near these clip points that the noise comes from. So, depending upon your car, it would be common to hear this noise coming from the driver side, center, or passenger side. Mine was coming from both the passenger side and center, with the two noises having different pitches.
I've attached a couple of photos showing where I greased this joint, taken from outside the car. The Shin-Etsu worked like a charm and I now have total silence! I don't like the idea of using WD-40 on the dash because of the solvents in it. I think the Shin-Etsu will both last a lot longer, and may even "condition" the vinyl and plastic parts enough for it to be a permanent solution.
Hope this helps anyone having the same problem.