Just a SPST, single pole single throw relay, ie very std type common one around £3 or so, few lengths of wire, fuseholder near to battery, terminals to suit, then just run a live to switch, then back from switch to relay to trip relay, then output from relay to fans, beef up the earth to the fans at same time.
If you want overide switch simply tap another switched live onto relay "trigger" position.
tabetha