I removed my right front wheel, then removed the bottom section of the wheel well liner. It's the way the Caddy's are done, but I don't know if yours has the sectioned well liner. My antenna was held in by two screws accessed from under the hood (very easily accessed) and a third that is also under the hood, but further down (not as obvious, but accessible). I believe the top is held in place by the escutcheon, but mine was missing. Not the easiest thing to get to, but I've seen worse. Something my service manual recommends that I didn't do, but should have is to send the old antenna down with a 4-ft wire and you can use the wire to pull the new one back up the fender into place.
Cool color. Parents bought an '86 Estate wagon new, it was silver over grey pillowed cloth. Really nice car except for the 307. Learned to drive in that car. Welcome! -Mike