I was reading a Chilton manual on this, and I agree with Complex that the flow to the heater core is sometimes valved off if you have your A/C set at max. This would be bad. You could tee into the heater core return, as long as it is after the heater core. I would not to the bypass line. The cooling system bypass always flows, it just flows less when the thermostat is wide open, allowing flow from the radiator. This might be restrictive as well. The bypass temperature is what tells your thermostat to open when the coolant gets hot. I would tee the supply off of the heater core feed, then return to the main return line. Stay away from controled lines like the heater and return. I don't know where exactly you could tap or find plugs, but they are all over the place on the engine. Might take some research.