OK, if the left turn signal light flashes on the trailer when you hook it up I would pretty much think it has to be the bulbs or sockets are bad in your truck. I'd put a new bulb in and seee what happens before anything else. If no help, pull one of the bulbs and put a test light in there and see if it flashes.
It could be the bulb(s), or possibly a faulty connection but if all bulbs are ok, it's probably the turn signal.
You can keep ignoring that apparatus
Replace the flasher.
You probably have a bulb out. Remember, the rear has two blinker bulbs
Did you replace the flashers? if not, that's the problem. They are located on the right of the fuse box, two round metal things. About the size of a large botle cap. Runs about 6 bucks for both.
have you checked your fuses?
Optical sensors convert light, or changes in light, into electrical signals. They are used in products such as automatic lighting and flashes for cameras.
When I drive, the turn signals work fine when I'm not tilting the steering wheel. So if I were you, I'd get your car checked out.
Defective flasher relay.
Could be the flasher unit.
Check the rear ground wire. Make sure it's properly connected to the vehicle frame.