It's almost like the CAN bus isn't sending the data.
It sounds like it needs a diagnostic read, but you can do a basic check on the CAN bus yourself. If you have a digital multi-meter, you can measure the resistance of the 2 CAN wires in relationship to each other. They should have a termination resistor which should have a resistance of about 100 Ohms. I believe the CAN data bus for the trailer module starts at the Ipack, which is where the terminating resistor is located.
In answer to the question about direct connection to the vehicle electrical system.
It's possible and has been done, but I've also heard of reports that the extra load on the drive FETs (all lights are driven by FETs) causing them to fail, which means a replacement CJB is needed to solve the problem caused.
Me I'd get the trailer module doing what it should, but mine was easy and worked straight out the box.