Assuming this isn't a console car, I'd trim the back of the bezel and install a AM/FM/CDRW/MP3 head unit. The mod is free and the head should be about $120.
Add a pair of 10" subs in sealed boxes, and a small amp, and a pair of 6X9s in the package tray, not to mention a dual speaker setup in front, and volia! That's what I'm doing, but since I have a '66, the radio isn't going in the dash!
HTH
--Kyle
You know, I have never thought to use that space, but its a good size and you could hack up a repop piece for next to nothing. I have my amp and eq mounted in the glove box at this time.