In order to get flow through your heater, you will need to make connections to the engine in locations that give you a pressure differential between the two connections. Usually, if you connect one to the thermostat housing, the other would need to be connected to the side of the block, or at least in a parallel flow path to the radiator.
You will also want to install some sort of shut off valve in the heater loop, so that you do not rob flow away from the rad during the warmer months when you are not running the heater fan.
HTH - SR