I'm running dual Ford Contour fans. Both on high draw 40 amps total when running, and the starting surge is an unknown amount larger. I wired them both to come on full through a large relay driven by a temperature switch.

When they'd both kick on, the extra load through the alternator would drop the idle 150-200 rpm and sometimes start to spiral in (big cam won't idle below 1000) requiring a touch on the throttle. Annoying with a manual trans when (for example) backing up...

Anyhow I'd recommend doing it the way Derale recommends - or better yet, get a beefier PWM controller from Darryl the Auto Cool Guy. I love mine, well worth the extra cost. And no high load surges either.
(No connection, just a satisfied customer).