I had the same problem a couple of days ago. On my truck I used a swagelock to tee into the hot line. I used the grounding post to the right of the cooler line to ground to. I just soldered a wire to the body of the fitting. Works great but, I think there is a reason the cooler is not grounded (electrolysis?) the same reason the radiator is not grounded. That would make sense why there is the rubber hose inline to isolate the transmission cooler from the grounded transmission. I might check this by seeing if the radiator is grounded after I grounded the line.
I might be buying a new line ($12) and putting the tee on the transmission side of the rubber hose.
I dont think that telflon tape keeps every bit of thread from touching, especially on a thread that is designed to jam on (NPT). Mine had continuity with an adapter and the sender covered in liquid telfon.