Hi Mafradon,
I haven't worked on this for quite some time, so I might not remember all the details correctly. The problem is that if you want to use the internal GPS, you need to get access to the virtual gps com port that is normally blocked by navi.exe. In my hack this is done by reloading the virtual com port driver, but this causes navi.exe to lose the GPS data stream. So with this method it is not possible to have igo and navi.exe working at the same time even if you use a portsplitter, because you cannot get navi.exe to reopen the GPS com port. The only way would be to cook a new winceimg.bin such that the portsplitter gets loaded before navi.exe. But this is dangerous, because you can easily brick your HU.
Pioneerus works in a different way, i.e. it reads the NMEA sentences via undocumented Pioneer API calls and feeds them to a virtual com port driver.
Alternatively you could use the debug port on the back of the HU to connect an external GPS mouse.