Sharing a couple of lessons learned so far since I didn't see these spelled out in other discussions when I searched on P0720 and on Gearbox Fault.

To get to the 18 pin transmission connector JB155 I removed the battery and battery tray, then without jacking up the car I reached from underneath and was able to push upward on a 10 pin and the 18 pin connectors that come out of the transmission to release them from slide holders on a small steel plate. Others have mentioned removing that steel mount plate by removing 3 x 10mm headed bolts. That works too but I didn't find it necessary. Once the electrical connectors are loose from the steel plate they are easy to access from above. JB155 has a slide lock. Pull and/or pry it up about half an inch and the connectors pull apart.

The body of JB155 that goes to the car as opposed to into the trans has markings molded on it to show pin numbers. See attached, the pin numbers are on the back of the connector, the black molded part and not the inner white part of the connector that holds the pins in.



The wire colors in my car do not match the Electrical Guide for Model Year 2005. I see this mentioned in other posts but I'm a bit of a lunk head so had to draw it out for it to make sense for me. Diagram attached. Note these are simplified so if a wire is, say green with a black stripe I just mark it as green for quick visual ID.



Only pins 3 and 5 are different colors from what I find printed in the manual.

The pinpoint test for code P0720 in the service manual directs you to measure resistances from pins on JB155 to pins on JB131 at the Trans Control Module. My car doesn't have a JB131 because I have the 32 bit TCM. A close look at the wiring diagrams led me to this translation for 32 bit TCMs:
Service manual "measure resistance from JB155 pin 5 to JB131 pin 42" = "measure resistance from JB155 pin 5 to JB231 pin 26"
Service manual "measure resistance from JB155 pin 6 to JB131 pin 5" = "measure resistance from JB155 pin 5 to JB231 pin 29"

Of course in the middle of all this I found what is probably the real fault, corrosion in the TCM connector. I'll address that and knock on wood and cross fingers and see if the gearbox lives.