Need pictures dude.
Try to lay everything out so we can clearly see the wiring.
What I mean in my previous post is, as well as the video wire from camera to video tx it needs a ground wire from camera to video tx to complete the circuit.
This will be common with the battery ground.
Make sure you don't back feed live from the 3s to the tx, you will kill it.
Also check the pin diagrams, do not rely on the wire colours when you hook everything up.
So, to recap, the video wire that used to go to your old camera, (usually yellow) goes to your new camera.
The ground wire that used to go to your old camera (usually black) goes to your new camera and also to battery neg.
The power wire that used to go to your old camera goes to new camera (usually red, 5v, not much use for 12v camera) disconnect from tx, hook to 3s battery positive.
Other wires if you have them are probably audio.
Once again,
check the pin diagrams, do not rely on the wire colours when you hook everything up.
The Chinese have been known to get the colours wrong.

I hope I haven't over simplified things, I'm just trying to be clear.
Good luck.