Transmission leaking into radiator?

Absolutely the right answer from the mechanic. The transmission has a cooling line that goes to the radiator and back into the transmission again. The radiator has corroded (it is 11 years old and they usually have to be replaced after 10 years or so) and it is leaking into the transmission, replacing the fluid with water, which will ruin the transmission if you don't get the water out of it, especially if you live in a cold state where water freezes in the winter. Get the radiator changed, the transmission drained (and a new transmission filter installed) and you should be good to go, those are great little cars and will last a long long time outside of normal wear and tear like this....

sounds like you have A bad Radiator to me, there are Transmission lines that connect to the lower half of your Radiator to cool the Transmission fluid, hot fluid will cause the tranny to slip. Gotta flush the coolant system, after replacing the Radiator you should also service the tranny.. just my opinion

cheap fix:

take the cooler lines off the radiator that come from the tranny...

you need to plug/cap those holes.

you now just need to connect the 2 cooler lines together.

a decent auto parts store can get you what you need for $20 or less.

you'll need to add tranny fluid as the water evaporates out.

you should take it easy with the car...no towing or trips up hills until this is properly repaired....but it'll work a long time as a daily driver.

This sounds VERY strange

radiator coolant and transmission fluid

are not near enough in the way things are put together

as to allow for mixing. Please ask your mechanic for a

DETAILED explanation and if they can not provide one

or the explanation sets of B.S. alarm(s) ... find another shop.

GOOD LUCK