Heating in Vauxhall Vectra 2L (1996) electrical problem?

Vauxhall Vectra 2L (1996) - The car is in fact an Opel Vectra (living in Switzerland so it's an Opel) and is owned by a lady with not much to spend on fault investigation.
The car intermittently provides no heat into the seating area. Was taken to a garage and she was told that the fan is intermittent and he suggested that she take it to an electrics expert. He temporarily rigged the fan to run all the time. She was told to keep an eye on the temp gauge and to pull over if it gets hot. All of this cost her 100 quid, and that's before the fault is even diagnosed.
I would appreciate any information that could lead to a fix on this. For info I'm not a mechanic but I'm not clueless for rectifying the fault myself ... if I know what it is.
Has anyone got any ideas?
Thanks in advance.
Paul.

Paul Davies
January 2008