1975 C3 Roadster

The high speed does not work.  Switching 3 speeds do, the last position does not.  I replace this switch 2 years ago with an ac Delco.

Could it be the high speed blower relay?

Is there a way to jump the relay to ascertain if it's bad?

Comments are welcome, thnx, Phil

That sounds like the blower motor resistor in the engine compartment.  It is mounted on the plenum and sticks into it for cooling of the relay.

This might help:

https://www.corvettemagazine.com/1963-1982-corvette-blower-motor-troubleshooting-and-repair/

4 wires on the high speed blower relay. Orange, purple, dark blue, black. Orange is 12v feed from blower fuse. Purple goes to the blower. Dark blue is relay signal from blower switch, black is ground. Shoot 12v to the dark blue terminal, the fan should run high speed. If it does not run, the relay is either not grounding, or bad. You can also just check for 12v on the blue wire with the key on, blower switch on high. Voltage there with no high speed points to relay/ground problem. NO voltage there points to blower switch.