[Tom Wells]

Rugby,

Don't know anything about BDR or its wiring.

Having said that, logic tells me there's a possibility that the thermal switch is wired incorrectly.

Here's why: you replaced the thermal switch so that should eliminate the switch itself.

Then, the fuse blows when operating temp is reached, which is when the thermal switch should activate.

If that does happen, it seems possible that the thermal switch is connected to a ground and not to the fan so when operating temp is reached, the thermal switch may short the 12V feed to ground instead of to the fan?

My SWAG for the day,

Tom

PS: On my cars the thermal switch is connected not to the fan, but to a relay. So the thermal switches I have do in fact ground the 12V signal through the relay, not the fan itself. No idea if yours is supposed to be done that way or not.