Ground the headlight switch to the body at the steering column hold down bracket. The tail lights are looking for a ground path. Most headlight switches on these cars actually have a spade terminal on the switch housing. the switch normally grounds through the mounting in to the dash bezel. Over time this ground path becomes unable to support the current flow. Corrosion , too much paint , and lack of use. Remember it's the headlight switch housing that is to be grounded