You disconnect the two rubber hoses going to and from the cooler and connect them together with a fitting fitting!
(a pipe and a couple of hose clamps)
That way you lead the water past the cooler, and you can see if the leak stops. Drive it for a while until you are secure that it makes a difference.
If it does, chances are good that you have found the culprit.
If not, you can reconnect it and look for alternatives. Or leave it off. Chances are good that it will develop a leak in due course.