Fonzi and Rumb are leading you down the right paths.

The early models had an "anti-theft" top. You use a slotted key to pull up the antennae if it were fully depressed. A small electronics screwdriver works also.

There is a removable panel/splash plate in the inner rear fender-well. Many times the screws have been covered over with either rust proofing or road gunk. Should be two on the bottom of the panel and two on the inboard side of the plate. I believe they are 8MM hex head sheet metal screws.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now