1974 Duster /6 rear lights

Do the front park lamps work? If so that fuse is good. Does the tag lamp work? If so the park lamp current is going through the harness to rear of car. I've seen a crushed or otherwise defective tag lamp fixture blow the park lamp fuse repeatedly.
Turn signal switches do fail to interrupt turn signals and brake lamps in a variety of manners. Left and right turns work but brakes don't, One turn works but other and doesn't and/or one brake doesn't, etc...
A lot of owners will replace the cancel cam alone which is only about half of a turn signal switch. That can be short lived fix. Lets not go there yet though.
If the rear bulb sockets are metal, they have only 2 wires. 2 contacts in bottom of socket. 2 contact at base of 1157 bulb. One is park lamp, same color wire both sides and at tag lamp. Other is turn, different colors for left and right. The ground path for any and all begins with bulb base/side wall to interior of the socket ( corrosion there has been fault found for some ). Ground continues through the socket fit into a metal fixture, the fixture to body, and finally ground jumpers from firewall to engine block, and radiator support to battery neg. The one rear turn working says there is some ground path there.