I took some masking tape and drew the section where I wanted the stereo reciever to go under the mirror on my 90 prostar (same model) and used a dremel tool to cut the opening. It worked awesome but made a big mess!
Here is a pic of someone else on here's mc with the stereo mounted in the same place as mine. Right under the mirror. I wish I could find one of those Mastercraft Reciever covers like in this pic.