If the transmission cooler in the radiator fails then coolant and transmission fluid mix causing the dreaded "pink milkshake" in the transmission. 99.9999 percent of the time it leads to total transmission failure and replacement. A good rule of thumb is replace the radiator every 10 years or 100k miles if you decide not to bypass the in radiator transmission cooler and use an external cooler. Keep in mind that the radiator can visually look good on the outside but you have no way to inspect the transmission cooler components in the radiator. They are subject to a lot of vibration which seems to be the main culprit.