My Interstate was missing it's radio and a matching filler plate was installed when I got it.
I purchased a JVC CD unit and mounted it in the trunk and purchased a set of speaker for the rear from Wingstuff.I also replaced the front speakers as well, even though the ones in there worked. They had white colored cones and just looked odd mounted in the dash.
I had to modify the rear speaker housings to get them to fit and wired up everything myself. I have a remote attachment so I can control the volume and use my Sirius unit from my truck mostly!