Just a couple of quick adds, more details later:
The failure occurs at the junction with the power cord and pad. It may be possible to rig up more support for the power cable and avoid the failure but that area also gets a lot of road water. Perhaps using the adhesive to anchor the power cable to the pan. Mostly I think it is 'cheezy.'
The small hole on the side of the drain plug was an attempt to make a small sample capture system. Unfortunately there was too much contamination from the plug to make usable samples and the oil seeped too quickly around the loose threads ... darn it.
This is the other side of the pan showing the thermistor installed for oil temperature measurement. What I'd like to find would be a pass-through, insulated terminals with gasket. Then the heater element could be inside the pan, affixed to the metal to warm the oil in a uniform manner.
I would also recommend getting a copy of:
ORNL/TM-2005/33 - Report on Toyota Prius Motor Thermal Management
ORNL/TM-2004/247 - Evaluation of 2004 Toyota Prius Hybrid Electric Drive System Interim Report
I just wish we could come up with a 'live steam' heater. Some sort of cover that we could flow steam from a heater against the transaxle pan. It would transfer a terrific amount of heat, very efficiently.
Chat at you later,
Bob Wilson