First off you'll need to have the video module back in the rear quarter panel where the CD Player also goes. Also your Nav screen will have to show the Television option. From there in my 97 540i I was able to turn the TV on very easily. If you don't have the TV module, perhaps check
www.navtv.com they should be able to set you up or buy the module at BMW store.
Watch TV / Videotext while car is in motion:
Set ignition key to position 2 (where fan, A/C, etc. are on, but you haven't started motor)
Enter TV mode
Enter "Settings"
Enter "Contrast"
When the green contrast bar is visible, push the knob and keep it pressed.
Turn ignition key to "OFF" position (while keeping button pressed).
Wait 8 - 10 seconds (accurate timing is important)
Release the knob.
Turn on ignition again.
Now TV will display while car is in motion.
(Obviously, you'd better have at least three eyes if you do this very often - one on the road, one on the rear view mirror, and then you can devote one to the TV.)
I originally printed this from "Tom's Homepage: Repair Infos BMW 750i", which was at
www.750i.de/repair.htm. I think this page disappeared once, but it is reachable today. You should bookmark that one!