You could add a temperature controlled switch in series with the fan's power line..
Attic fan thermostat 85F thermal switch solar electric - eBay (item 190527871313 end time May-28-11 21:53:37 PDT)
I've used these type switches on heat-sinks to kick on the fan when they got over 100 deg F.
You might be able to find something similar in an automotive supply store..