[Safari Larry]

Great start! Pictures... Exterior shot, dash, under hood might be helpful and interesting. Good progress on electrical testing. Something to implant firmly in your mind, "NEGATIVE GROUND".

The Motorized antennae is an uncommon option and an indicator that there might be several options on the car. It is fortunate it is working. Radio not working is likely a bad tube. That can wait. Horn, radio, & lights work with ignition off. You can test directional signals and by turning the ignition on (doesn't need to be started).

For dashboard lights, turn headlight knob all the way to the right and see if they light up. For horn, press the horn ring. If it doesn't work, put it on your list of things to fix.

For removing the gas tank, you might find the first video linked by Sirrotica (thanks) to be helpful where he removes the tank. I watched most of the first video. It was long and addicting! Very interesting and helpful to someone putting an old Pontiac back on the road. I believe there are reproduction gas tanks available for your Star Chief. If there is a lot of rust in the tank, a replacement might be a good option.

Fuse box is under dash to left of steering column. Fuses are labeled.

Onward...