Normally the Nav Plus will mute the television picture whenever the car is traveling above 5kph. There may be situations where you want to defeat this.
You should always be cautious that driving while watching TV is dangerous.
This assumes you have already installed the TV tuner, or other video adapter. Basically the Nav Plus mutes the television picture when the car exceeds 5kph. When you exceed this speed, the screen will display the message "For your own personal safety the TV picture will only be displayed when the vehicle is stationary.".
Understanding the GALA signal:
The Nav Plus detects when the car is moving by using the GALA or speed signal. GALA apparently stands for "Graduated Audio Level Adjustment", though I think it really is something in German. The GALA signal is used for two things. As the car's speed increases, road noise typically increases. The Nav Plus system will increase the volume slightly as the car's speed goes up. As well, the GALA signal is used to improve the navigation accuracy. When the antenna loses signal, such as in a tunnel, the speed signal is used to "dead reckon" the car's movement, based on the car's speed.
One option is to simply disconnect the GALA wire. On some systems, this makes the navigation update very slowly. The audio adjustment will no longer work. It's also possible the navigation will be slightly less accurate, especially at low speeds.
The second option, which I chose, was to install a switch that would allow me to turn the GALA signal on and off. This sort of allows the best of both worlds, though you do have to push a button. How I made the switch is listed below.
Model Specific issues:
Some users with older cars don't bother connecting the GALA/ABS signals. If you haven't connected them, and you are not having problems with navigation update speed, TV will not mute.
The latest A4 Audi's use the CANbus signal for speed instead of a GALA connection. This includes the B6 version of the A4. You have to use an ABS signal for the Nav Plus. Removing this signal will cause the Nav Plus to not mute.
How to build a switch:
Really this is pretty easy. You will need an "on/off" toggle switch somewhere in your car that simply "breaks" the GALA wire going to the Nav Plus. Flip it off to watch TV, and on when navigating. The GALA is connected to two spots on the NAV Plus, connector 3, which is the lower 8 pin connecter, pin 1 and the 26 pin connector pin 5. I am not sure which actually does the TV muting. I simply interrupted both.
I did a slightly fancier mod. My A4 has a "blank button" to the right of the fog light controls. I replaced this button with a switch which allows me to turn the GALA signal on and off. You can see the switch in the picture below, between the fog light controls and the emergency button.
In some cars this is an ESP control, so this mod won't work for everyone.
The blank button is simply a filler, not a switch. I couldn't find a switch that did not have something written on it, so I bought an ESP switch, part number 4B0 927 134 B98. I then "pried off" the ESP button part, scraped out some plastic on the back of the blank button and put the blank button on the ESP switch. I now have a "stealth" TV disable button.