IIRC the air con system is a bit of a night mare on these cars with regards to wiring.

First off, get the air con system serviced. It'll cost you in the region of £80 and it should be carried out annually.

With the heater, it could be possible that the thermostat is stuck open. It should be closed when cold so check that it is.

Also check that the heater matrix hasn't been bypassed as sometimes when they start to leak, people can't be arsed to repair it (then sell the car in summer)!!