I just had a similar problem with my '92 SSE */c. I recently noticed that the HUD control wasn't working. When I moved the position control up or down, it slowly returned near the center position all by itself, as if there was a gooey spring inside.
I could only see the HUD reflection in the windshield if I leaned fully forward in the seat and looked as far forward near the base of the windshield as I could. I didn't need to open the moonroof.
I removed the lower dash cover but to my dismay I wasn't even able to see the HUD control cable behind the dash because of all the other stuff under the dash. So I removed the front dash panel cover and removed the 2 torx screws for the HUD control panel, and I was pleased to see that the cable was still connected but it was binding in its sleeve and was slightly bent. That explained the slow auto-return behavior that it was doing. I carefully bent the cable straight, then lubed the exposed area of the cable and sprayed some penetrating lubricant into the opening in the cable sleeve and I was then able to adjust the HUD up and down without any problems. Boy was I glad that I didn't have to disassemble the dash or remove the instrument cluster to get to the end of the cable that connects to the HUD mirror.
I hope this helps.