sdujolo

I have a audi A4 is it possible to read out if you have the reverse gear in? And can you read out steering controls so you can use it to control your computer? And can use send info to the screen where the normal radio and car computer writes info?

bbalazs

Reverse gear light is usually swithed in the gearbox. Two possibilities:
The reverse light is powered, but not grounded and when you lever the reverse it is grounded. (I found this in my former car Fiat 126p)
OR
The reverse light is grounded, but not powered by 12V. The 12V line is interrupted by a switch that is moved by a lever somewhere at the gear box.
(I found this in my current car Fiat Seicento 1.1)
Regarding these, you need to 'drain' the wires that lead to the reverse lamp, and according to the possibilities above, you can detect the 12V on and GND off or vice versa.
If you want to detect it by a TTL-compatible circuit, use a 4.7K serial resistor AND a 4.7V zener diode toward the ground.
In my car I apply this method for my microprocessor to switch the reverse camera signal to the TV by a relay. When relay is off, the signal from the computer AV out is on the TV.
Other possibility when you have a monitor that sets automatically to the AV input when it provides signal. In that case the rear camera is wired directly to the reverse light.

I am not sure, but some steering controls are working with resistors by two wires. The resistance tells you, which button(s) are pressed.

The third question I think is difficult for a DIY project.