Left flasher dead, right on works

No voltage. Replace the turn signal switch. All the exterior lights run threw it. Except the rear high brake light.

Has the vehicle been in an accident on the left rear side. First check the bulb to be sure it is not defective, trade it out with the side that works properly to test. If the bulb works well then My thought are you may have a shorted wire. The turn signal flasher works by heat build up within the flasher the heat is caused by the load of the bulbs, the load heats the metal spring causing it to open the circuit then as quickly as it cools down the circuit closes again lighting the bulbs.
There is also a possibility of a defect in the turn signal mechanism on the steering column. The turn signal switch on many models does three different things on many models that use the same bulb element / or bulb for brake and turn on rear of vehicle. It controls the circuit for brake light by sending power both left and right side rear when turn signal is in neutral position then when turn signal is in left turn the switch cancels brake light power to left side rear and supplies flasher power to left side while maintaining brake light supply to the right side rear, and so on opposite when the right turn is chosen on turn signal switch. The turn signal indicators on the dash normally run off the same circuit as front turn signal bulb only which is also controlled by turn signal switch. Front has turn signals but no brake light.
I think there is an additional load on the left side rear turn circuit and it could be either a wire that is shorted or the turn signal switch if the same bulb element is used for both functions.
Often times if trailer lighting is connected the flasher will also blink a little faster because of the extra load of additional bulbs.

see all indicator bulbs it only happens when one or more of the signal bulb fails to work

the next step is to check fo voltage going to the signal socket and check the ground, the flasher clicking faster means you got a light not working

possibly flasher unit faulty
or turn signal switch

a very bad earth in that lamp holder,, clean all the earths and try another new buld in it, iv had this before and that was the problem sorted

Normally when this happens you have a blown bulb on either the front or rear the fast blink is because with only one bulb working the flasher clicks it faster less of a load means faster blink.

If the right side signals are working properly and you can hear the signal flasher clicking when the right side signals are selected, the flasher is not the problem. Its more likely you have a short to ground in the left side wiring somewhere.

You have a broken wire to the turn/brake light portion of the light as they use the same wire. Or where the plug goes into the light assy could be bad. Torsion mount lights are horrible and you should think about replacing with a universal light that you can wire direct.

sounds like the bulb you have is the wrong resistance the new noen bulbs will do this