Pictures of the aftermarket antenna? Lots of older aftermarkets went on similar to a toggle/molly bolt in drywall. Others just had one leg under them that would grab on to the metal of the fender, and you could loosen the base and worm it out through the same hole. There should be enough room to get your hand up in there though, through the backside of the fender in one of the large openings.
Factory style fender mounted antenna is the cable, a base that is screwed to the fender with three screws, a trim cap and then the mast itself. I can upload photos tomorrow of the factory setup, I have one laying on my bench at work ready to be installed on my 87, get rid of the horrid windshield antenna.
http://www.lmctruck.com/icatalog/cc/full.aspx?Page=164 in case it helps any. Their picture really does not resemble the factory mounts that I am used to seeing on the 85 and up squares.