this is for MK1 nav (male voice), but may work with others:

set Ignition to position II
enter TV mode
enter 'settings'
enter 'contrast'
when green contrast bar is visible, push nob and keep it pressed.
Turn off ignition
wait 8-10 seconds (timing must be accurate)
release nob
turn on ignition again.

You should be able to watch TV while in drive now. Crazy, but it works (for some older e39's). It worked for mine a couple years ago and has survived a couple engine software updates.

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Watch TV / Videotext during driving 

This works on all 5/7 Series with the Mk. I  Navigation System (the one with the male voice). On Mk II (the newer version with the female voice) you have to use "brightness" instead of contrast for TV and "color" instead of brightness for videotext to get the trick to work. I know that the trick works on latest 99 7series models as well. But I heard also that some cars sold into the US do not have any TV equipment on board. You will notice this as they do not have any TV option in the main menu. Also latest BMWs sold in Europe (2000 onward, 3/5/7 series) have the trick disabled! The only chance to get the TV running is somewhere hidden in the menu of the BMW Sevice Tester DIS. But up to now I can't thell you where. If I ever find it out, I'll post it here! 

As far as I know the software update for latest navigation CD Roms does not affect or disable this trick. 

Set ignition to pos. II 
enter TV mode 
enter "settings" 
enter "contrast" 
when the green contrast bar is visible, push the knob and keep it pressed 
turn ignition off (keep button pressed!) 
wait 8-10 seconds (timing must be accurate!) 
release the knob 
turn on ignition again 
Now you can watch TV during driving (but always keep at least one eye on the road - that said) 
To watch videotext use the same procesure as above, but chose"brightness" instead of "contrast" (thanks to my friend Herbert Posner, who discovered this secret).