I think you'll find it is more a matter of whether the manifold heat or the car heater works in the colder weather. By tee'ing them together the water will flow through the path of least resistance, most likely the manifold heater and your interior comfort will suffer. If you are attempting to tee them as you describe I would put and adjustable restrictor ( valve ) on the line coming from or going into the manifold heater. Since the manifold will not need anywhere near the volume the cars heater does you should be able to throttle it down to a point where the cars heater is satisfactory for comfort.
