For my passenger van without rear heat from factory. I bought a rear heater unit from a wrecked 15 passenger van, ran a couple of new 3/4" hoses from the aux heater to the rear midship location (T-connectors were already factory installed on my Sprinter in the engine compartment right behind the radiator, but I chose the aux heater since it was already there), I mounted the unit under one of the seats, mounted the 3-speed fan control switch I took off the wrecked junk yard van on one of the side wall panels, wired it to a new relay and fuse and ran it to the second battery and I have very hot air for the children now. I mounted the switch in the rear area of the van so the kids can control the heat as they need to, I don't even have to bother with that as I did in my conversion van, where the kids would constantly tell me "it's too cold" or "it's too hot back here".
At first, I was worried the Sprinter engine would take a long time to heat up, like my Cummins, which takes 10 minutes of driving in winter, but the Sprinter heats up really fast. My van did have the aux. heater installed from factory so I spliced the hoses into that instead, but I guess anyone can hook up the rear heater right up to the engine heater hoses up front, which probably already has a Tee connector installed. Mine has a couple of caps on it, since it's still not being used.