I used a sheet metal screw. It was pretty small, but I don't remember what size. If you have the under dash AC vents it'll be harder to get the screw in than if you don't. Without AC you can just reach under the dash with a screwdriver.
I have AC,but the vents are missing and so was the radio. The previous owner probably pulled the vents off to get the radio out. Still looking for the 1968 vents too with brown vent pad.
