I have an Emprum UltiMate GPS that cost me $100.00 online, comes with iPad app program for software support. It fits my iPad, iPhone & iPod old plug.
I only use it when on trips since I have the iPhone for land based travel. Even on trips it is only 5-6 trips so I don't really use it that much, just another cool toy. The GPS sucks battery power so you will need an on board charge system. That extension cord looks like a good idea, I may spring for one even if it is only 3' long, it looks like it will fit in the hole through the case I have, one of those 'Survivor' military style covers.