The one on the right is the return line to the engine, it will be cooler since it has been through the radiator. From what I have found out (and the way I have mine) you tee off the drivers side to the heater input and the out put tee's into the return line on the passenger's side. I'm sure this is how the factory is setup, now one other thing is where you have your heater mounted, if it is above the height of the radiator you might need to a small pump to help with vapor lock.So just to be clear, come off the rubber hose closest to the radiator then? I was plummed in first on the rubber radiator hoses right of the engine and that didnt give much heat.
If this is the one it's the same one I used, I put mine under the hood. I didn't think I could fit it under the dash, I would like to see how yours is mounted.It is a heater from Summit Racing, I think I paid around 155 with shipping. Summit Racing® Automotive Heaters SUM-991106