I have a 86 toyota pickup with almost no mods to it electrically other than a radio that was installed over three year before i bought it even. Anyways shortly after i bought the truck the turn signals completely stopped working. Easy fix. Went to the junk yard and snagged a flasher out of another 86 pickup and everything worked perfect for a while. Now comes the wierd part. I use my turn sigal switch to go right, everything works perfectly up front but when i go to the rear, the left is flashing and both brake lights flash as well. This happens when i turn the headlights on or if im on the brakes. But everything works fine when neither is on. I tried a new combination switch, same problem. Took off my box and followed all wires and resoldered all the connections where they split to go to the two tail light connections to get better continuity, now all the lights light up brighter which i suppose is a plus but still the same problem. Does anyone know if the flasher could cause a problem like this?

 

Also, it does the same thing when going left as well, just so i dont get asked. And all my bulbs seem ok.

 

Do the front turn signals light when the brake pedal is pushed??

Does this have a converter box for trailer lights??

Could be you have a poor ground in the rear lights turn signals grounding into the brake light circuit.

Clean the sockets with a fitting brush and electrical cleaner

another possible thing is damage to the wires which has the brake circuit and turn signal circuit touching.

 

^^ what Wyo said. And I have seen bad bulbs do some really funky things. I've chased circuits and re-wired portions of circuits, only to find a defective bulb...

So just as a baseline, I would replace all bulbs, after cleaning all sockets well.