Turn Signal Troubles

If we are working on an A-body, Im guessing its a problem in the turn signal switch itself. And in front left a possible wrong bulb or wrongly installed bulb, or bulb with bridged filaments. In post #4 you say the front stops flashing: does it stay on brightly or go out all together? Also, are all these test done with the park/clearance lights off? Does the vehicle have a trailer plug? Electronic or standard flasher? Hazzard flasher?
The signal switch has several functions. Its supposed to keep the fronts separated from the rear and the lefts from the rights. If you have a bridged filament in the front left it would send power into all the clearance filaments thus turning all the bulbs on.This would explain the rears on and the front going bright. However since all this power is coming through the flasher, it should be flashing frantically. Unless its electronic in which case it would be flashing normally. For the bulbs to remain lit the power has to come from a non-flasher source. That leads me to suspect a cross-feed in the switch itself, either from the brake circuit or the hazzard circuit.
Since all the bulbs light to normal brightness, I believe all the chassis wiring and grounds are ok.
I would check all the leftside bulbs for correctness and correct installation, then swap out the left front, and disconnect the brakeswitch, then retest with parklights off. If problem persists I would inspect wiring on column and finally the T-switch itself. If the problem goes away with the brakeswitch disconnected, test with parklamps on.If the problem returns, check the wiring. If the problem doesnt return go to the t-switch.
I think this is right. But check that left front 1st for bridged filaments(you could swap it to the right front to see if anything changes).I really hope this helps.
I have had funny things happen when the t-switch melts inside.