Essentially, it would be easiest for you to wire another relay in parallel with the OEM driving the fan, then trip that relay with a toggle located somewhere convenient to you. It becomes a "Logical OR" - the fan will run IF the OEM relay OR the added relay is energised (or both - that's also the way of the Logical OR.)
You'll need:
Relay
Toggle
12AWG wire (for power to the relay and power from the relay to the fan)
18AWG wire (for the signal lead from the added switch)
Some sort of "side tap" or "wye" connection to the fan - probably best to use OEM-style connectors, and just make them with a "wye" arrangement, so you can feed power from either relay to the fan.