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The correct location is under the passenger side. There are 2 type of heaters Ford put in. THe first used the air in the cab and was called the recirculating heater. All it had was just the 2 hoses from the engine going to it. The second was the "Fresh aire" heater. THis had a air duct coming in from the passenger side kick panel that fed the heater. It got the air from a tube and duct assy that was above the tire and bolted above the headlamp to pick up fresh air. It also had a water temp valve that bolted to the firewall above the heater. If you look at your firewall inside the engine compartment, you'll see the knockouts for it. The control had 3 cables as the recirculating had only 2. Your heater is a aftermarket. THis was very common. Sears, J.C. Whitney, Montgomery Ward and others all offered heater kits to install. These were usually cheaper than Ford, so many opted for these instead. That's why you see so many with oddball looking heaters in them. The factory heaters come up on Ebay now and then. You just have to keep looking. The Ford shop manual has excellent diagragms of the heater set ups.