As you can see there are many wires hidden under the harnesses insulation that go to several different connectors. The only way I know to get a TRX scooter running with its original parts is to replace any damaged or missing parts and harnesses with new original parts and harnesses.
I wish that I had a better solution than just throwing money at new parts but since TRX does not provide a detailed wiring diagram that is the only way I know to fix them and keep them original.
Alternatively to using the original parts you could custom install a universal speed controller and wire it to both motors to get the scooter running. This would involve removing or not using the sensor switches and the original controller and wiring harnesses and custom making a new simple wiring harness for the scooter. I am sure that without the sensor switches it would turn easily in all but the tightest of corners.
We have all of the parts and harnesses available for the TRX Personal Transporter so if there is anything that you need just let me know. Also, please let me know if you have any questions.