The glass roof was an option, both factory and dealer installed. The proceedure is involved and centers around removing the metal moon roof adding different brackets/seals etc. There was a service bulletin describing the process. Finding all the parts could be difficult. There are also two special tools refenced and the bulletin allowed 3.5 hrs of labor to install.
The bulletin starts ou by telling you to verify that you have all the parts and going on to discard the metal panel and the sliding roof panel. Bulletin 280 Group77 dated july1992.
I dnt know if I would attempt it unless you have parts & tools