Do you have to hook these up to a manual switch or can you splice them into the stock wiring so they come on just like OEM fans?

 

Most fans I've seen draw over 30A. It would be unwise to use just a switch. I would use the OEM harness to fire a 40A relay circuit that you install to power the new fans. Or use the OEM harness but be sure to use good heavy duty means for splicing.