1985 745t

I picked up a new/used aux elec. fan from teh junkers on saturday, tested it on the battery (works great), and am getting ready to slip it in. However, I have a question about rewiring it to have it go on whenever the radiator temp exceeds a certain amount (regardless of speed or ac activation).

How exactly do you rewire this so that the fan will come on whenever the temp exceeds a certain limit? Do I need to buy a secondary sensor or can I use the existing one and just rework it somehow? Is the temp sensor in the middle of the hoses (on the turbo its located at a junction between the hoses right behind teh radiator) used for anything other than the aux fan? It triggers the heating system, right?

Basically just not that skilled reading electrical diagrams and looking for advice from someone who has already dealt with this issue. I could buy a fin positioned temp sensor at autozone and wire it between a hot lead and the fan so its on its own loop, but I'd like a more elegant solution that modifies the existing system if one is available.

Any help is greatly appreciated.

Thanks,
Rick