A volt ohm meter, or a test light, is a helluva lot cheaper than buying parts, and throwing them at a car, only to have changed everything that bolts to the electrical system, and find that corrosion, or a loose connection is causing the problem. As a rule, I never change anything electrical until I've isolated the problem to that part, saves me, and customers money, and grief.
I've seen more than a few GM cars that the small wire (purple) that energizes the starter solenoid is either is loose at where the nut is not tight enough for a good connection, or has corrosion under the terminal.
If the wiring is original it can be in the bulkhead connector connector where it passes through the firewall. Any wiring though could be suspect if it's original to the car, but even new cars have wiring problems occasionally.
That's how I solve electrical problems after twisting wrenches for a few decades. YMMV...
