It would be exactly the same as a 1957 Chevrolet. Wrecking yards are your best bet but remember only the 1957 Chevrolet unit will fit. All Canadian Pontiacs came with the top of the line Heater setup that was available for the Chev and Pontiac that year. In the U.S. the Chev also offered a cheaper recirculating heater setup as well, that was never offered in Canada. Keep in mind that the American 57 Pontiacs had completely different heater setups. Some of the minor parts for this setup is reproduced. All the big stuff is not. Obviously you have to find a parts car that has all the required parts. I would be very surprised if you do not find the whole setup here on this site through someone on this site. Good luck with the search. Cheers. George.
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1957 Pontiac Pathfinder Deluxe sedan restored 261 six
1974 Chevrolet Caprice Estate wagon low mileage original 400 V-8
1956 Pontiac Pathfinder 2dr sedan, 496 - dyno'd 545 hp, stick shift, 4.11 posi - Hot Rod