For that model of Jensen you dont need to hook up the ebrake wire to the actual brake all you have to do is ground that wire and you can use your dvd whenever you want however dont use it while your driving!!
As for the backup camera it sounds like you wired the camera to the reverse feed but not the actual head unit. The camera needs to be wired to the reverse feed and also the head unit will need to be as well. The Green/White wire coming off the wire harness for the radio needs to be wired to the reverse feed, this is most likely your issue and will switch to the backup camera when it is in reverse! Let me know if you need any other help