Tranny oil cooler is factory installed.
It's the small radiator look-a-like near the radiator.
Many people misunderstood that it is engine oil cooler.
Engine oil cooler locates at the base of oil filter bracket.
Believe it or not, Toyota does call that little loop on the tranny case an oil cooler, although it's hard to believe it's very effective. I wonder if there wasn't something in the WRC rules limiting oil cooler placement to the stock location and they put that loop and ducting in place so they could install a larger one in the same location for racing.