Marc, sorry if I may have tossed out a red herring.
I'm the kind of old-school curmudgeon who likes doing things the hard way just to be ornery.
Almost all normal people use a thermostatic switch to control the cooler fan and that's what I'd recommend for you - unless you just like being ornery, of course.
But @Michael McKelvey raises another point that's now been mentioned a few times here - the 'Mocal sandwich'.
This deals with another consequence of installing an external oil cooler. You don't always want all that extra cooling. In cold weather, you want the engine to reach normal operating temperature as soon as possible - and before you start using the engine really hard.
With an external cooler connected (and no 'sandwich'), it can take as long as a half hour for the engine to warm up if it's in the 40s or below. (Being from California, I've only read about such things in books, but it's my understanding that this 'cold' phenomenon can happen with some regularity in your part of the country.)
The 'sandwich' is a mounting plate (with its own thermostatic valve) that diverts oil around the cooler until the extra cooling is needed, so your engine gets up to normal temps much quicker. 'Mocal' is a popular brand of 'sandwich', but there are others. Gordon will probably be able to give you an in-depth explanation of how all this works.
Again, being ornery, I don't need no stinkin' sandwich, but as long as you're going to the trouble of putting in a cooler and you experience this 'cold' thing often, you should give the sandwich some serious consideration.