Quote Originally Posted by w2zero
Hot switched wire to relay, relay to fan and 12v source. Bourdon tube to radiator and fan controller, to relay. Twenty bucks for the adjustable fan controller and sensor. A few more bucks for the relay and wire.

Balance that against when you will forget to turn it on or off with a manual switch and either smoke the charging system or blow out the cooling system and maybe lose the engine.
Your right. Not going with manual switch. I know how that would go, you nailed it.