I have seen some, but just my few questions:
How would you stream audio signal to car speakers?
My first mind is to put somehwere aftermarket stereo with bluetooth audio support. Aux input is sh*t and you would need external amp for it.
How about turning it on off, charging etc? Ipad battery could die fast.
I have seen somewhere a holder for ipad for mk5 but the price was around 200 quids custom made.