Through a leak in the ATF cooler embedded into the radiator.
If you look at the bottom of the radiator, inside the engine compartment, there will be a metal line on each side, that goes to the automatic transmission. The solution is to change the radiator. You may need a special tool in order to release the ATF lines if they are not normal threaded fittings.
Worse yet is that antifreeze can also get into the transmission, and would require changing the ATF.
Through a cracked transmission fluid cooler that is internal / integral to the radiator.
You may have residual trans fluid left in the engine if you didn't flush the engine out.
through some rubber hose.
