heayer blower motor on 1991 Jaguar Vanden Plas

heater blower motor does not seem to work on any setting. i turn on the defrost and the blower motor comes on sometimes for about 20-30seconds. whats my problem and what can i do to fix it to gain additional heater control!

in my case it was the blower motor itself. the bearings had lived their life and died. seems that more current is used on defrost setting and this then overcomes the binding bearings. I had to replace the fan. try connecting a 12v hot lead to fan and see if it runs and if you hear bearing noise.