Thanks for all the advice. It has completely died now (position four doesn't work) and even the air recirculation switch is dead. I took the heater resistor out and none of the connections are burnt at all.
I was lead to believe it could be the switch itself which is causing it to completely die in addition to the problem described by philvbarnes. I seem to remember it was very stiff, almost stuck on position 4 when it completely died and I had to force it out of position four. Although it has completely died before and come back to life.
So I pulled the radio out, removed the front panel and removed the switch. It has six connectors on it. I got the multimeter out, set it to measure resistance and with the switch set in the zero position I joined connector one with two, then one with three, etc then repeated the process for each connector. None of them were joined. I then repeated this with the switch in position four and got the same results.
Was this a valid test? Should I expect that when the switch is in position four there should be some connection between the connectors?