How do I remove the tail light lens cover so that I can change the turn signal bulb in a 1991 GM?

 

Not sure about a Grand Marquis, but most rear light replacements are done through the trunk either by directly removing the socket and bulb from the back of the housing, or you might have to remove the back of the housing. Once you get to the back of the housing from insdie the trunk the rest of the job should be self-explanatory.