Your fastest way to finding the issue is start at the fan and work backwards.
Find the relay that operates that fan. It won't be the one you don't like to access inside the cabin. It should be up front by the light bar. Trace the wires from the temp sender to the relay. Once you find the relay, you could manually trip the relay to see if the fan starts. You could also supply 12v directly to the fan to insure it is working. Let us know if you need more detailed instructions to accomplish these tasks...
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