My 63 C172 stall horn doesn’t work. I see the circuit is on a 2 amp thermal breaker. It also sends power to the Attitude Indicator. Why would a vacuum AI need power? Trying to find out where I have a power loss. Thanks.

The circuit breaker SHOULD send power to the stall horn and the turn and bank indicator, unless someone has done something weird to your plane in the last 57 years. If the turn and bank works, check for voltage on both terminals of the stall horn. Then check for voltage on one side of the switch in the wing. Basically current goes thru the horn, the switch supplies a ground, so when the horn is quiet you should have voltage everywhere except the final ground in the switch. hope this helps.

Yup, what Dale said. It's common that the little micro switch in the wing quits working. They are very expensive, so be gentle with them. If you have voltage out to the switch and it's not making contact you can give it a good spraying with electrical contact cleaner. Try to get the spray up under the tab into the switch itself. Then work the tab up and down, gently, and hopefully it will clean itself up if that's the issue. Also check and make sure the ground wire is making proper connection.