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.
indicator
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.
You probably have a bulb out. Remember, the rear has two blinker bulbs
Optical sensors convert light, or changes in light, into electrical signals. They are used in products such as automatic lighting and flashes for cameras.
have you checked your fuses?
Check the rear ground wire. Make sure it's properly connected to the vehicle frame.
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.
Turn signals just quit working- have replaced flasher- checked all bulbs and fuses, checked all obvious wires, cleaned grounds, replaced the turn signal switch (that was an adventure). At a loss- any help out there? Hazards work fine- all other lights work fine- turn signals dead
In Project Management, a risk trigger is an identified measure or indicator that signals to the project that the risk event may occur.