[VetteVet]

After thinking about this some more I realized that the purple wire is hot any time the key switch is on. This means that the signal indicator light would also be on and it would flash off when the turn signal flasher blinks.

Wiring a couple of small diodes into the yellow and dark green, L and R, rear turn signal wires, would prevent feedback and would flash the signal indicator light whenever the L or R turn was selected.

The only glitch would be that anytime the brakes were applied the signal indicator light would illuminate. To avoid this you would have to wire the signal indicator light into the front turn lights so that only one of them is on at a time.
The front signal light wires are usually light blue and dark blue.

All I have is Chevy diagrams with the two lights so I'll have to look and see if I can find one for the GM. You've peaked my curiosity now.

There is another possibility but it would mean that the turn signal switch for the GM trucks is different than the one for the Chevys. The Gm switch would need to have separate contacts for the indicator light with separate wires to the light. Checking the part numbers for the two switches should show that.
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