I'd say do exactly that, but instead of wiring to the fan switch, you might consider going straight to the relay. Then you'd get all the benifits of the above, plus the added advantage of switch failure not crippling your fan switch circuit. Switch failures aren't very common (but not exactly rare, either), but if it does, your system will operate normally. You'd only need a single pole/single throw switch, as well.Use a single pole double throw switch:
Pull white wire off of radiator breaker and connect to L1. Crimp another wire onto blue wire connector of radiator fan switch and connect to L2. Common would be connected to white wire (12V) out of the radiator fuse (spliced between fuse and breaker). So when switch is up, circuit is "normal"; when down, you've jumped across the temp switch and apply 12V to the fan (fan on). This keeps all your fuses and breakers functional for safety.