[VetteVet]

Disconnect the half moon connecter for the steering column TS wiring. Connect a multi-meter on the white wire and the dark green wire. With the meter on the ohms scale, check for continuity between the two wires, Zero or very low ohms. Be sure the TS lever is in the neutral position.

Now move the lever to the right turn position and observe the meter. It should go to OL or not reading.

Now move the lead on the white wire to the purple wire and the meter should read zero again. It shows power has moved from the right brake light wire to the right turn signal wire and it has shut off the right brake light.

Now we test the left light.

Connect the meter to the white wire and to the yellow wire and check for continuity. The TS lever should be in the neutral position
The meter should read zero ohms or very low like the right one did earlier.
Now place the TS lever in left turn and observe the meter for OL or no reading.

Now swap the lead to the white wire to the purple wire and the meter should read zero ohms. Showing power to the left turn wire.

This is normal operation, now we'll check the brake light cutout.

Connect the meter leads to the white wire and the yellow wire. with the lever in neutral the meter should read zero ohms. Place the lever in right turn and the meter should still read zero ohms. No brake light cutout. This is the point where you are saying that the left brake light goes out when the right turn is selected and if you don't get a reading then the light is not getting power through the TS switch.

To verify you can move the test leads to white and dark green wires and with the lever in neutral you should get zero ohms. Move the lever to left turn and the meter should still read zero ohms showing that the right brake light is still on.

There are several small contacts in the turn signal switch and it is not uncommon for them to get dirty and lose connection. Often pulling the steering wheel and the horn button and cleaning the switch with electrical contact cleaner will restore it's functions.

The wiring color code is sometimes different on aftermarket TS switches so the wires may not match the colors on the truck harness. but there positions will be the same.

I think I have this correct. Test your switch and post back with the results.