electrically troubleshooting A/C/heater switch

[gramps77]

Has anyone got a method to troubleshoot from the A/C and heater panel to verify where a problem lies?
I can shoot wires just fine if I have a step by step (IE, in this switch position you should see X volts)
The problem Im having is a replaced my HVAC panel with one from LMC and it appears the switch is junk. Reason it was replaced was the faceplate was cracked and the lettering was wrinkled and in bad shape and the switch before I thought was bad but it looks like it was the relay.
The new LMC one use to work in only the lower 3 settings and now if I manipulate it can get it to work in only high, the lower 3 no longer work. The thermostat assembly that goes in the box on the engine side gives the same result at the slider switch regardless if I use a new one or the original, so I assume its the switch. I WAS going to replace the whole system with Vintage Air but looks like I can save some bucks. Thank you