1975 honda tail light problem (electrical)?

Hi there, ive got a 1975 honda cb500t with a tail light problem. So, the tail light will not turn on. The brake light will though(same bulb). Everything else electrical also works. When I turn the key to "run", the headlight and running lights illuminate, but no tail light. When I turn the key to "park", which is supposed to only illuminate the tail light, the main fuse and tail fuse blow. I took apart the main ignition switch and cleaned the contacts, but it didnt help. any ideas?