See IDOM4 manual (search web for ID-O-MATIC manual)

I use this on my repeaters as:

RED wire to common power supply for this fan relay board and your IDOM4 
BLACK wire to pin 4 of IDOM4 (program that pin in IDOM4 programming as Menu 4, item 7, "C" . Then remember to "W" write the changes back to the IDOM4 board.)
Connect your fan to whatever power supply that powers it. Cut ONE of the wires to the fan, connect each of those wires, one to each black mark terminal on fan relay terminal strip.

When the IDOM4 gets a valid signal in from your receiver, the fan relay closes and runs your fan for 2 minutes (programmable in IDOM4 menu). So, if someone keys up your repeater, the fan runs for two minutes, if the repeater is in use for 5 minutes, the fan runs for 7 minutes and so on.