LOL, if any antenna mounted under dash receives AM more than a couple miles from transmitter, I'd be surprised... BTW, just noticed my '96 F-150 has a rectangular base antenna, apparently same size as the later '60s & '70s Ford vehicles(yeah I was scraping that damned white stuff)... One thing that may differ is the base may not be same thickness at each corner, some are angled to fit contour of fender...
I apologize if I'm not understanding that "Huh?" emoticon. Does that mean you don't know/never checked to find out, or does it mean you don't understand my question?
I was just curious because I know someone who has one of those "hidden" antennas under the dash, and it only gets FM.
I have always found that the black "gasket" type insulator is different under the base depending on what car/truck it was going on.
It looks like the same one that is on my 70 Mustang rectangular base and all. can be had at most Mustang parts houses for around $20.
I got one at the local auto store. Was square. I eventually just had the whole filled in. I gutted my AM/FM radio and hid put a mp3 hookup in it.
Assuming your fender is drilled for a original rectangular base, the screw holes will probably still be visible...