Or, you can bag the Toyota system, and for another $1000 (

), get the latest car-based PC from Azentek.
http://www.azentekonline.com/cms/content/view/41/146/
I just read about these from cnet.com and pcmag.com, who are both covering CES 2008 right now. It's a full Intel dual core PC, with a 120 GB hard drive, and a touch screen. It's got bluetooth, and wi-fi (skype/VOIP calls?), a PC card slot for mobile broadband internet, built in GPS, a 4x50 watt amplifier, AM/FM radio (satellite ready), plus a USB port in front, and a DVI output in the back. Not to mention the OBD II and CANbus interfaces available. It's a double DIN unit, which means it should fit in the opening where the NAV unit would go.
I'm pretty sure there are other manufacturers as well. This might be an alternative to the factory/non-factory debate.