Nav confusion Please help

[Ramseyd]

Long story but I'll make it short. Bought e53x5 with nav that will play radio but no display just blank screen, seller bought another unit and gave it to me with the truck and I replaces the old one with this new one and same thing.... I searched around and to my understanding you can test is the nav is bad or good by going in the rear and unplugging the TV module and plugging that blue plug into the nav module and it will then allow your nav unit to display and work just as it should of the unit is good. Well o followed all these instructions and I have no TV module at all. I can not find it I have everything apart in the rear and found no module. Why? Do some not come with a module? I need help. I'd like to at this point take the easy route and get a after market nav installed but then how do I know if my two stock ones are good so I can sell them? What should I do..