As far as I know Chevy had their own heaters in recirculating and also a fresh air heater. The recirculating heater was a smaller unit with a round core in it, the fan was a blade type and blowed air through the core. It didn't draw outside air in. The fresh air heater was a larger unit and bolted up to the inner cowl to draw air through the vent on the side of the cowl. It used a rectangular core and a squirrel cage fan to draw air through the core. smile