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?

 

your various lighting systems are grounding throught your tailight/dashlight circuit. you have a bad ground on your rear lights to repair somewhere

 

This is a shot in the dark, but search the entire vehicle for loose ground connections, especially inside under the dash. Just lay on your back with a flashlight and search for a wire that connects to a bolt that is installed in the metal structure. I had an '88 S-10 that was doing really strange things like yours is. I lived with it for a couple years. One day while working under the dash, I discovered a loose ground connection. I tightened it and never had the electrical problem again. I bought the truck new, so it must have slipped by in the factory.

 

check the ground at the frame back at the left rear...some ******* probably pulled the bed to do a fuel pump and left it loose