If you have a "Heater Booster" on your Sprinter, it is an Espar Hydronic unit. It is located, just forward of and below, the starting battery. You can tell if you have one by looking into the left front wheelwell and seeing if you can see a small squarish muffler - that's the Espar's exhaust.
The heater booster is to be activated (in 2006's and earlier, via the left end of the center bar switch, below the A/C switch, in the dial furthest to the right by the passenger seat) in temps less than 40 degrees, in order to keep the engine coolant, in what is a fairly cold-blooded engine, at normal operating temperature (180 degrees I believe)
In city type driving (stop-n-go, stopped and idling) the engine will not produce enough heat in cold ambient temperatures to fully combust the fuel properly and will run "cold" - which is "bad" - because it will contaminate the oil with unburned fuel faster. Running at highway speeds this normally isn't a problem - the engine will stay warmed up.
I suppose it is possible to heat the engine and the cab with a Hydronic - although way it is implemented on 2006 and earlier units, it does not - at least not without the vehicles engine running. Part of the problem with doing this and using the cabin heater blower motor is that it likely would have a fairly high amperage draw.
If you are primarily concerned with cabin heat, you are far better off IMHO just getting an AirTronic - it will probably cost less and likely use way less fuel (particularly if your Sprinter is well insulated) - because it isn't also heating an engine which is exposed to the cold - which will act as a huge coldsink (opposite of heatsink) and therefore require more fuel to operate.
