[AZKen]

First change bulbs, Bulbs have been known to do a feedback thing if one filament of a two filament is burnt out...and other weird things.
The power to the TS switch is from the flasher, then the switch distributes it to the proper bulbs. Since you have the rear working correctly, it's seems like a front bulb or ground problem. Bad/lost ground can do the same weird/feedback things that a bad bulb can do. My guess is that it is NOT the switch IF you have not messed with that area or it's wiring or any wiring lately.

Ground the bulb sockets to a good frame spot to test while bulbs are flashing. Also: even though a bulb's filaments are OK, it's base pins can loose ground inside the socket. Corrosion!!!! Clean and lightly sand socket and bulb pins. I think Canada has an environment conducive to corrosion/oxidation/rust.

90% of Truck electrical problems are bad ground connections. The frame, body, motor, sockets, are in the path to battery negative. They are connected together with jumper wires, braid, cables and intimate contact.