Fan switch inside car for heat does not work

Hello, no that's not a good idea, it's never a good idea to run power wires which are not build into the vehicles systems. You can use a 12-volt automotive test light to check for power at the blower motor. You'll see there's two wires going to the blower motor. A purple wire and a black wire. The purple wire should have 12 volts on it with the blower on high and should light up the test light.
There's also a Blower Motor Relay which energizes the circuit and a blower motor Resistor block that limits current flow to change the blower motors speed. Since the age of the vehicle, its most likely the blower motor or the Resistor block have failed. I'll put a wiring diagram below so you can see what wires you're testing. As well as a guide for using a test light or multimeter to diagnose the problem.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/blower-fan-motor-works-on-high-speed-only