I just checked out the Tekonsha tranny temp gauge installation instructions.
They show that it should be installed downstream of the cooler, in other word on the return line where the cooled fluid is going back to the transmission.
I've always been of the opinion that gauges should be installed as close as practical to the heat source, before any cooling takes place, since that's when your oil will break down.
So, I would install the gauge inline with the hose going from the tranny to the cooler, as close to the tranny as feasible, before any cooling takes place.
Any thoughts on this?
--Kamus