I would guess that this is a PWM (pulse width modulated) signal so you can get variable brightness. If so, the pulse width should increase as you turn the dash illumination brightness up.
On the Blaupunkt CAN adapter, there are 4 DIL witches to select which radio you want to interface with. Have you tried changing the setting to see if this makes a difference?
How did you plan to interface the illumination signal to the CarPC?