I've found something really cool while browsing the service manual

You guys are going to like this!
1. Turn the ignition switch to ON (II). Make sure turn off the audio unit.
2. Push and hold the preset No. 1 and No. 6 buttons. While holding the buttons, push the power button (A). Release the buttons and the self-diagnostic mode begins.
3. Press Preset button 5. Now, when you drive the vehicle, the VSP indicates the vehicle speed in km/h.
4. The self-diagnostic mode ends when you turn off the audio unit or turn the ignition switch to LOCK (0).