Left turn signal flashing rapidly

If all the lights function normally, I would suspect that there is high resistance in the left side circuit somewhere. The trick is going to be isolating it. For starters, does the vehicle have some sort of trailer wiring? Next would be to look for something that may not be too obvious, but may give a sign. Is there any difference in bulb intensity (brightness)? You could have a socket with a bad ground and it is feeding back through another circuit. I cannot say as though I have seen a vehicle with a rapid flash on one side, and all lights functioned normal. There is no service procedure for that symptom in the service manual. When did this occur, or when was it noticed?

It started happening shortly after we bought the truck, used. We put in the new flasher unit, and the flashing increased, which we both thought was weird.

There is trailer wiring I do not know if he has checked that out yet, but he is usually thorough, although this has him really frustrated.

We have a manual with the electrical sycamatic in it.

Hubby is electrical engineer for most part, and has tried everything that he can think of short of pulling the wiring in the truck with a voltage meter. Will tell him what you have suggest, he said that he had been in the truck, under the truck, under the dashboard, almost inside the glove box.

Could the unit that the flasher is plugged into be having an electrical issue?

You would think if it was as central as the flasher, you would have issues on both right and left. If they were both flashing fast, it would almost seem normal. If the trailer wiring is not factory, it might be suspect.

I had this problem it ended up being the turn signal switch. FYI

When I turn my right turn signal on it blinks at a rapid speed. When I turn my four ways on it is back to normal. I have put all new bulbs in and it still does this. I checked the fuses and they are all good. The grounds are grounded. Also my reverse lights do not come on.

Check grounds at left and right rear frame rails. Then have it scanned for codes it may be turn signal module or body module problem.

If it changes anything, when I turn my right turn signal on, the license plate light will blink dimmly. Right brake light also does not work when I apply the brakes.

That sounds like a ground issue. Check the one at right rear of frame like I said in first reply then check battery ground/ body and engine. See picture.