Engine dies when you turn on the aircon or the heater

Im not quite sure what bypassing heater coil has to do with aircon leak, unless you mean there's coolant leaking from under dash, if coolant is leaking, remove hoses at firewall and follow them back to engine, you can use one of them to bridge from one to the other. Or put a connector between the hoses

Turn the heater tap to hot setting.

the engine doesn't like the extra stress of the air con unit being run _ there is no way of fixing this _ unless you get a full engine overhaul or a secondhand engine and installed _ the engine is old and the extra pressure on it with the aircon it just cant handle it

Sounds like the aircon is kicking in when you turn on your heater. This will load up an idling engine, and if it is idling too slowly is will stall out the engine. Try increasing your engine idle speed until the heater on function doesn't stall the engine at idle speed.

There is a clutch between the compressor and the engine that turns the compressor over. Although unlikely, if the compressor has completely seized then when the compressor clutch engages it will stall your engine.

Your car should have an aircon on/off button. Turn your car on and immediately switch off the aircon, if the compressor clutch is at fault it should stay disengaged and you should be able to use the engine.

the heater core has a leak in it and yes it can get into the aircon system as its in the same system

When the aircon is switch on it puts a substantial load to the engine: you need to get your car tuned to take into account the load from the aircon. Most car have a mechanism that increases the idle speed when the aircon is active: if you are unable to get your car tuned yet, try making the engine idle speed slighty faster while you have the aircon turn-on (the ajustment screw should be close to the accelator cable connection at your engine end). Don't make the idle speed too high and check by turning the aircon off and on to assess the best compromise idle speed to stop your car from almost stalling (e.g. jerking).

Good luck!

May be your heater/AC control unit in the dash if you have already replaced the mixing valve for the heater.Must be a problem. I just responded to the same issue for a 2004 Focus.

The idle is controlled by the idle air control valve, an electronic valve fitted on the inlet side of the engine, they get sticky and cause the car to stall. Sometimes they can be leaned and lubricated but in most cases a replacement is required.
To prove it is this valve, with the engine idling start turning on as much as possible, lights, stereo, heater, aircon on full blast etc.. the car should pick up on revs as these things are activated but your car will probably stall due to the valve sticking.

Is your aircon working okay? The aircon should kick in to dry the air, with the heater in defrost mode. Hope this helps..