I do know the OEM Toyota Navigation has a seperate module that is located in the rear of the truck, but I don't think these units are the OEM (Eclipse) units. I think these units are just named OEM, my suggestion if you want nav is to choose what is proven vs trying to re-invent the wheel to save some money at the offset just to learn you need to add a lot later.
My choice: Eclipse AVN6620, it can be found on ebay for $700.
It is a plug and play unit (at least for my truck) I took out the stock radio, plugged it in, stuck the gps antenae above the unit and voila 30mins later a working unit. It lets you use steering wheel controls without any extra wiring & or parts and needs no new wiring harness to splice in.
It does however not keep (if you have it) the oem line input in the center console. (If someone knows how to make that work please tell me).
Everyone will have there own input into what unit is best, I've used every manufacturer now (Alpine, Kenwood, Pioneer, Eclipse) and found each to be very good, it's really to me just which one has the features you want. I really like the simplified wiring of the Eclipse unit there is not a lot of space behind our dashes for a lot of extra wiring or additional pieces.