left signal light stays on
 

Whenever I turn on my headlights the left signal light turns on solid and will not flash when I put the signal switch on. The right side operates fine all the time. Here are the rest of symptoms.
When I check the actual signal lights they are also both on when the headlights are turned on.
When I turn the head lights off, the left signal works perfect.
When I apply the brakes, the signal left comes on the same way it does when the headlights are turned on and the dash lights glow real dull.
The common thing seems to be the rear left parking light and or brake light. When I apply the brakes (headlights turn off and engine off), the left turn signal light slowly lights up (front and rear). When I turn on the 4-way flashers it seems to operate, but the buzzer goes on/off rapidly, which is not normal.
I replaced the rear bulbs but no change. Checked the wires going from the sockets and underneith the truck (as much as I could see with the spare tire in the way) but nothing seems to be shorted or worn.
Any input would be greatly appreciated.
Mystere