IF you are getting the trigger voltage (which is sounds like you are being it does switch for a second) from the turn signal indicator assembly you may have a bad video selector box. All of the cameras plug into it and then the selector outputs the camera to the monitor. Good luck and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7