You didn't say which Transit you have but if its a RWD with a 90 or 120 PSI engine you need to check the fusable link under the bonnet, it is located just next to the red plastic "+" terminal where you would put the jump leads if you had a flat battery. The main power supply for the heater plugs takes its feed from this terminal and if one of your old heater plugs shorted to earth it would blow this fusable link, it looks like a piece of wire but smaller in diameter than the main supply to the heater plugs and bolts on to the aformentioned terminal, unscrew it and check it by giving it a tug or check it out with a multimeter.
Adrian, July 2012