[68RT]

Normally most of those antennas were held in place by a nut threaded onto the antenna base, Once removed, the antenna was lowered slightly and a pivoting metal piece was pivoted 90 degrees making it small enough to lift out of through the hole. They usually had a rubber gasket and plastic on top of it to protect the paint. Either one has failed and the nut may be stuck to the base. Try soaking it with penetrating oil to break the nut loose.

 

[chuzz]

If that doesn't work, maybe you can remove the fender liner and get to it that way.