hey new guy, welcome to the garage!
we'd be better able to answer the specific question if you could give a link to the item in their online catalog, but if it's an OEM style unit it mounts inside below the glove box [below the battery tray, but inside], and any replacement unit I've seen mounts [like the heat/AC ones] the same place
most of the repro vendors [check the links page here] sell complete heaters for the Task Force series trucks, including some that may have used originals - the other thing you could do is make a post in the parts wanted section here for an original unit, lotsa folks here with xtra parts
Bill
