[ctuna]

Maybe its coded to L7, fiberoptic output.
you could test for an audio output by using a small speaker , meter or head phones.
The pro head units for the US cars up to 09 are suppose
to be programmable if it is you might be able to program it for
base or stereo ouput which would give speaker level outputs
without the need for a amp.
I wonder if the people that put your aftermarket deck in messed with the programming in the car.
Since I have never done the programming I don't know the details
there is more info on coding sub board forum.
US Head Units were either Hi Fi or L7 only before an up to 09.
(to my knowledge)