Since there aren't any C10 Colts in any way shape or form near my region of the country, I decided to grab a rear parcel shelf and speaker mounts from a Hyundai Excel. I know this is a Mitsubishi Precis and not a Mirage, but they share the same undercarriages. So I figured, eh why not try it. After a little cutting that you don't see unless you really look for it, and cutting of the brackets themselves, it works! I'm going to upload the pics a little later this evening.
I also obtained some professional light grey carpet/vinyl/plastic dye and redid the parcel shelf, and painted the mounts silver to match my car.
Well if anyone needs a quick solution to having rear speakers and some privacy, this works great!