So, no TV module...

the aftermarket units could be hard to troubleshoot.

Lets establish, what you already know about this setup -

do you know where the unit is located? (I would suspect, in the trunk, but is it there?)

if you can locate the unit, you can disconnect the video feed from the reverse camera and connect your test monitor to the camera feed to see if there is any video coming from the camera.
then, you can connect your test video camera (or, could be video feed from your headrests), into the reverse camera input going to the monitor, to see if the video is passing that way.

in doing so, you will be able to cut in half the troubleshooting - either the camera is not working, or the feed into the front...

so... do you know where your aftermarket unit is at?