Lots of dodges, my Audi, and probably other cars do have a shutoff valve on the coolant line going to the heater core, but it's only activated when the air resirc door is closed.
VW went to an air blend door setup in 1984 for the Golf / Jetta, it allows quicker, more precise control of cabin temp (at least that's what the tech brochure said
)
2x on getting a coolant flow diagram. That will let you know if the heater loop is just part of the bypass circuit, or the only path from the bypass circuit.
-J