Thats weird!
I'm assuming though its a bit like the BMW system which, for factory-fit satnav, takes inputs (bizarrely) from the ABS sensors and elsewhere so that when it loses its signal, it carries on along the planned route assessing roughly where it is on that route from the speed/directional signals its getting...yer average TomTom just goes "Duh signal lost!".
There are speedo electrical connections on the gearbox somewhere I believe, or you could tap off the rar of the head unit, but as to which cables lordy knows?
What made you pick that unit out of interest? Its not exactly mainstream and the odd reviews I've seen haven't been especially favourable.....not looking to pee you off or be contentious, just curious as to what sold it to you? :thumbs