I might be able to get a 5 volt sensor signal to the fan by unplugging my relay under the dash and jumping power from my EFI sensor output. It's a long shot.
I do know that Andrewh's Mercedes has a multi-speed fan and it's the same style setup as these. It usually runs on a low speed until the A/C kicks in then it steps up.

I've got to mess with the hot rod this weekend because I lost my dash, tail, and brake lights. If I get the chance I'll try the fan with a 5 volt signal.