Yes, if you have a pre 88 760, the white vacuum line has to do with your heater. Though usually when we don't have vacuum to the heater control valve it is open, meaning the heat is always on. If you have a more recent model, the vacuum line to the heater control valve is grey. You have a myriad of vacuum lines under your hood, just check your heater control valve and see if the vacuum line is hooked up to it. Now, I have had problems, on northern cars, with the heater control valve rusting shut. Since you just entered winter, I doubt that you qualify for what I consider to be a northern car. Diagnosing heat is relatively easy...few components to deal with...thermostat, heater control valve, heater core, and climate controller (which I have yet to see fail and not provide heat).

Of course if you are low on coolant...you won't have heat either.

Chris