On my 65 the rear outer and inner lights are the parking lights and brake lights , the center lights are the back up lights.
The brake lights are bright until you pull the socket out of the housing , then , without the additional ground the lights will dim some........plug the socket back into its housing and it gets bright again..........seem normal?
Anyway , if I have the parking lights on and THEN hold the brake pedal in for a little while , eventually it will blow the fuse and I lose the parking lights , brake and dash lights and dome lights since they appear to be on the same circuit. Seems like a lot of lamps on one circuit . No I havent calculated the amp draw yet but the harness appears to be in tact and actually the wiring seems to be in pretty good shape. Bad ground?
Bulb technology (and numbers for the lamps has changed with time. You will have to add up the wattage of all of the bulbs and see how close you are to the amperage of the fuse (divide total Watts by 12 Volts to get the Amp draw). To find the Watts you can find that information written on the bulbs card in an autoparts store if you can not find it on line. It left the factory with 1157 and 1156 bulbs in the back,what is there now I do not know.