Ac and heating low blowing??

Well..........troubleshoot it..........

FIRST thing to do is get to the junction "right at the motor" and put a meter on there in "high." See if it's getting "full voltage" lacking that, run a jumper wire from a solid battery source to the motor.

Might also be possible the motor is so rusty it's lost grounding.

Otherwise, step by step check. With switch in high, check voltage on both sides of fuse. Should be "close to" battery voltage. If V is low at motor connection, but OK at fuse clips, time to dig under the dash, get your hands "actually" dirty, and yank the switch out of there.