if you're getting 5V evertime, it's not good at all !
that's why the tv tuner cannot display properly the reverse camera video.
5V, is in the middle of 0V and 12V, that may explain why screen's flickering!
You must get 0v (ground) when the reverse shift is engaged.
looks like your ground is not good...
try to connect pin 17 to an other earth
eg : ground of the rear tail light.
ps : just saw after that you're getting -5v on the photo !!
did you put the black wire of the multimeter on a ground,and the red wire on the pin 17 ??