You want to check that the "H" (headlight) terminal on the headlight switch has continuity to the Batt terminal on the headlight switch when knob is pulled out. This is done at the bench, No truck wires on the switch. You can test to see what other terminals have continuity to the batt terminals and to each other.
If you find that the "H" has continuity to "Batt", every time you operate knob, then the
switch could be OK as far as headlights. It means it may be a bad wire or terminal connection. Maybe a fuse in the circuit and is blown. Previous owners can do some weird wiring quality.
If switch test bad, be sure to do the same test on your buddy's donor switch before install. The switch terminal are marked with embossed letters...H, B, PL, T, S, D, etc. The Headlight terminal can sometimes be called "dimmer".
Normally those switches just need taken apart and light cleaning of copper contacts inside. Maybe a dab of lube on the slider. Check to see springs are not broken. You can stretch them a
tiny tiny bit. It takes three hands to get it back together.
EDIT
I say
switch could be OK because a continuity test does not load the switch contacts like headlight bulbs do. Weak/corroded/dirty/worn contacts inside the switch can carry a small continuity load but not a service load. So when installing the buddy switch, don't mount it to the dash. Let it hang down with wires connected and test it's operation. It's frame may need to be grounded when switch is hanging. Usually that's just for the dome light.
A new switch is $35....probably $85 by the time you read this.
