Tail lights

your brake light switch is bad.

Running lights (tail) are separate from the brake/directional . You need a multi tester to see what has power and ground and what doesn't . Old sockets can do weird things . Look under neath for an obvious problem , mice can invade even a daily driver .

If the lights hook up to a trailer, check that and back trace it all the way back along to the back of the truck cab. Also check the wire loom that goes from the engine to the firewall inside and make sure the connection is tight inside the cab. These tend to vibrate loose.