964 (UK model) electric cooling fan

My 1990 Carerra 2's electric cooling fan died (bearings) and eventually blew the 30A fuse in the engine bay fuse box. I replaced the fan and fuse only to find that it will no longer switch on and the car is liable to overheat. The relays and bi-metallic temp switch have been replaced and the temp sensors seems to be OK. Also the fan will switch on (low speed) as normal when the air conditioning or heater is used. If the first pole of the relay is manually closed (with my finger!) and the connector taken off the bi-metallic temperature switch the fan the second pole of the relay will activate and the fan will run at high speed, as it should. Anyone have any ideas what could be wrong or had a similar problem.

Any help much appreciated...the price of Porsche repairs here in the UK are phenomenal!!