If you only get cold air, it might be that your climate control switch is stuck, sending the cold air signal. My 1994 XJS has this issue and the mechanic diagnosed it as needing a new climate control switch.
thermostat stuck open or air in cooling system
I've a 1995 xj6 ,but where is the opening for the glove compartment? do it have one
fan motor could be bad
you have a bad heater core
that has a lot to do with.if your heater core is pluged up you wont get any heat out of it.
The common malfunction would probably clogged or resticted cat conv. Symptom would be no power, wont get up and go.
Piston heater element open.
Low coolant level? Cooling system airbound? Bad thermostat? Water pump not circulating coolant? Heater control cable not working?
Take it to the dealership, its a safety issue and is better left to the dealership to fix because if it isn't fixed right it is very dangerous for obvious reasons
bypass valve not opening
first make sure you have coolant in your radiator, and if theres coolant and its still not working check your thermostat it may be stuck shut and not letting water into the engine
check the distributor