Look down under your glove box. Move the heater indicator to hot and back to cold (blue to red) and watch the wire/cable move the little door, Does it move or is the wire/cable even attached?
Feel the heater hoses. Are they both hot? Does your defrost mode blow any air?
There are only a few things that will prevent heat from coming in to the cabin.
Air flow over the core, even though the PO said it's almost new...my son is 27 and I think HE is new!!
Heat, a.k.a. coolant circulating through, the core. This also can mean coolant level low, clogged core, bad heater valve.
Vacuum to operate the internal blend doors (the vacuum junction in the dash is behind the radio.
Vacuum to the whole HVAC system (reservoir, hoses etc).