What does “no good” mean? Does the right side not blink, or the left side or both? Is the flasher knob broken off? Does it not reset after making a turn? You realize that 1970 E-body is a unique one year only turn signal switch. Good luck finding an NOS one or even a used one.

If it doesn’t reset, the internal cam is reproduced(if that’s the issue). Sometimes just cleaning and bending the contacts gets them working (if that’s the issue)

EDIT: I've also seen the insulation cut on some of the wires on its harness, possibly shorting and wires completely cut.too.

Does your dad have the bullet terminal release tool? (The plastic terminal end will not pass through the column so the wire ends need to be removed from the plastic connector)

Hooking up a known, good switch is the best way to diagnose this issue.

With a brake light issue, it can be either the wire itself or the hazard part of the turn signal switch may have failed internally.

I ran into the same situation with a pal's Superbird, in that case, the brake circuit wire passed a continuity test & the issue was inside the switch.

Does your dad have the bullet terminal release tool? (The plastic terminal end will not pass through the column so the wire ends need to be removed from the plastic connector)

i just went through this with my Cuda ,, after I bought a new switch and two days of lqqking for a short I replaced both flashers. (There are two) id tried swapping the flashers with one another days earlier and it didn't make a difference, so I had figured they were both good??? (not) so if it hasn't been asked yet? have you checked BOTH flasher? a bad flasher will cause a lot of different problems! and I had a few going on ,