To get the AC going... the dash had to come out (to work the ducting and evaporator and all that).
I knew the body of my car was not in 100% perfect shape, but thought it was "OK for now".
Well, when the dash came out, rust was found at the base of both A pillars that was worse than expected - much worse. To remedy that meant the outside surface of the car would be affected (heat).
And since I was there anyway... figured I would just restore the whole car at once (rather than string it along in stages).
I am glad I did - just not something I had on the plans to do right then,