* Corrosion at the Transmission Cooler Line Connection: The most common culprit is corrosion where the transmission cooler line is connected to the radiator. Over time, the aluminum of the radiator and the metal of the cooler line (often steel or brass) can corrode, creating a weak point that leaks. This is especially true in areas with harsh winters and road salt. The leak may not be directly *at* the connection itself, but slightly away from it, due to the spreading of the corrosion.
* Radiator Tank Failure Near the Transmission Line Connection: The top tank of the radiator is a pressure point, and years of temperature cycling and pressure can cause cracks or weakening, especially near the fittings. If the crack is near the transmission cooler line connection, it might appear as if the leak is coming from the line itself.
* Damaged or Loose Transmission Cooler Line: While less likely to be the *source* of the leak if it's truly around the line, a damaged or loose transmission cooler line can contribute to the problem. If the line is slightly bent or damaged, it might put extra stress on the connection and exacerbate any underlying corrosion. A slightly loose line can cause vibration and leak from the joint.
* Manufacturing Defect (Unlikely, but Possible): While less probable after 25 years, there's always a small chance of a manufacturing defect in the radiator tank or the connection point that only manifests itself later in the vehicle's life.
In short: It's almost always a corrosion issue at or very near the transmission cooler line connection on the radiator. The leak *appears* to be at the line because that's where the fluid is escaping, but the root cause is almost certainly the degradation of the radiator or its connection point.
