I was going to install a transmission cooler on my '96 toyota 4runner but first wanted to know which line was the return line. I saw the pictures posted up and it showed that the rear line in the transmission was the return line but then I read instruction and they stated that the line on the passenger side of the radiator was the return line.

 

I used a multimeter with a thermocouple to measure the temp and figured it out that way on my truck. Althought it really doesn't matter that much whether you do the cooling before or after the factory cooler. My factory cooler only had about 5 deg difference between the input and output.

 

The rear line on the trans should be the return side. Just follow it up to the radiator. And, I would recommend you install the add-on cooler on the return side.

 

Thats weird....I have a TRD Trans Cooler and the rear line is the return and it runs to the driver side. Thats what it says in the directions, and thats the way it is on my truck. It says not to mess with the supply line. I traced it 5 times to make absolutely sure and the rear line (return) ran above the supply line and went to the drivers side.

I removed that and ran a new tube from there to the cooler and from the cooler back to the return. Thats the way TRD told me too do it. I dont know how similar your truck is too an 04 tacoma but I had to run the new transmission lines behind those supports in front of the radiator. So the it was the support, tubes, then radiator from the front. If I didnt the skids wouldnt go back on. Good luck!