My cooling fans have stopped working,i was told i

my cooling fans have stopped working,i was told i needed a relay switch for the cooling fans, so i bought one and can not locate it i have taken the left and right side behind the headlights out and no unit can be found. but when i put it back together and turned on the van they started to run and shut down like they use to and havent come back on since. could it be the relay or heating sensor or some thing else ?