Mark,
There was an earlier thread not too long ago with lots of pictures you might want to search down. You will want more of a blower than the engine fan if you're going to get heat into your backseat. If you just want to take the chill off the front seat, or maybe smoke the passenger's shoe sole, a manifold heater will suffice.
Here's mine, out of an old junkyard truck and painted up to match my interior.
Steve