Thanks for the info and the PID. I'll have to look into that one.Best bet would be to put your temp sensor on the side of the pan (to protect it from damage). The area on the driver's side of the trans, near the rear of the pan, is open. There are four (4) pan screws along the driver's side of the pan (not counting the ones just "around the corner" at the very front and back). The area from the 3rd screw back (between the 3rd and 4th pan screws, and beyond the 4th pan screw) should be free and clear.
But Sean is correct - there is already a thermistor for trans sump temperature (it's inside the solenoid assy). Might be easier (and better) to buy a scan tool that can be programmed to read specific PIDs. You can read the measured sump temp using PID B011. This will return a signed 2-byte value, that must be multiplied by 0.015625 to get the actual reading in °F.