You dont need to rewire the switch, you just need to do some engineering to make it fit the 73 bezel. You will not be using the original blower motor or wiring so that is not an issue. The kit is only made to fit up to 72. I installed one in my 73 so it can be made to work.
Have you sorted out the firewall plate yet? That was by far the most work of the install. I did not like their plastic plate at all so i made a metal plate for mine. I will see if I can find some pics.
Here are a couple pics. Also notice the plates I made to block off the holes where the original AC hoses and heater hoses passed through.
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