What would be the best way to electrically hookup an electric puller radiator fan (replacing the viscous fan) to a M104 engine with HFM?

I'll hookup the power with a 25A fuse to an unused fuse terminal, and a relay, but where should I get the signal from?

I can't "tap" into the "blue" A/C aux. fan temp. sensor because I measured 5V to ground.

Any ideas?

Thanks in advance,
:-) neil