I have a 90 chevy silverado K1500 stepside with a standard cab. This is one of the weirdest electrical problems i've ever seen.

first the tail lights dont work, when you turn on a turn signal, both lights activate like hazard flashers and the digital clock readout flashes in time with the turn signals. The brake lights work as long as the headlights are off, but when you turn the headlights on the brake lights(not the tail lights) come on.

I've replaced both the brakelight switch and the dash switch.

Does anyone have any idea what is going on here?

 

sounds like a short. or a couple/ fewof em. no fun

 

I had a problem like that with my blinkers on the dash turning on when I turned on the headlights, It was one of the rear bulbs. one of the two filiments broke and was shorting out on the other one. It wouldn't hurt to check your tail lights and your park lamps. Also, check and make sure that the bulbs are going in the socket the right way. One of the tabs on the base of the bulb is a little higher than the other but you can accidently force it to go the wrong way. Worth a shot

 

I would be checking the ground block located under the plastic cover on the passenger side of the engine compartment

 

I agree, check the ground connections, especially in the rear, at the lamp assemblies