This works on all 5ers and 7ers with Mk. I (male voice) navigation. On the Mk. II (female voice) you have to use Brightness instead of Contrast for TV, and Color instead of Brightness for teletext to get it to work.
Set the ignition to position 2
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 the ignition again.
To watch videotext use the same procesure as above, but chose Brightness instead of Contrast.