6 volt brake lights

I removed an aftermarket turn signal set-up on an old 1949 Ford F-1 pickup, and reconnected (I think) all the proper original wires. All the lights work correctly, except that when the tail lights are lit and I step on the brakes, the lights go out completely until I let up on the brake pedal. The brake lights work fine if the tail lights are not on. I have tried changing the wires on the brake light switch, but it has no effect. Any suggestions? I am sure it is something relatively simple, but I'm stumped.