turn signal problem

I hope someone can guide me in the right direction. I have a 99 GMC Sonoma that suddenly developed a turn signal problem. When the headlights are on the both turn signal lights on the dash are lite - When the blinker is engaged both front blinker bulbs will blink - the back ones are OK. When the headlights are off the dash turn signal lights are off and when the blinkers are engaged again both front bulbs will blink but the direction I am turning is brighter, and again the back ones are OK. I replaced the relay and that did not fix the problem - the parts store guys all pointed in the direction of the turn signal switch - so I replaced it and the problem did not go away. BTW all bulbs are good. Any help??

New development. Horn does not work - relay clicks - swapped relay with same result. When you hit the horn with vehicle on the turn signal lights on the dash turn on. I'm thinking this is all related - any thoughts??

It is common when strange and inexplicable things start happening to the electrical system that circuit grounds are at fault.

I'm not certain about your '99, but on my '96 there are several braided copper ground straps in the engine compartment along with all the wire harness grounding nodes.

Any damage recent or otherwise on the truck? Ground circuits would be suspect.

FIXED!!!

Thanks for the replies, they helped.

To make a long story short - I rewired a ground to the battery which in itself did not fix the problem, I then spliced a new ground to the horn and now everything works - I also did not realize that my parking lights weren't working which would have caused the turn signal lights on the dash to be on.