For example, many wireless access points use Linux, but you never see the source for those.
This seems to be Pioneer's GPL source code site: http://www.oss-pioneer.com/car/navi/files/ . I don't see AVIC-5000NEX there, but a variety of other AVIC devices are listed.
The GPL doesn't necessarily affect everything on the box, I'm not 100% clear on what happens if you sell a system using (for instance) GNU/Linux and put proprietary software on top of it, with regard to the source of the proprietary software. But you would have to make linux available.
The problems comes when people want to stretch the definition of separate programs. In those cases, contacting lawyers and seeking out the authors intent seems to be the only viable option (for example, the case with Linux modules).
I worked for some folks who were making an ubuntu based network-monitoring appliance a couple of years ago, one day they freaked out about the GPL and decided (absent any legal advice) to make us port the project to NetBSD. They refused to consult a lawyer because it would be too expensive apparently, but would have been far cheaper than all the developer time... I suppose at least I got to add NetBSD porting to my cv :)
http://gpl-violations.org/about.html
The news section mentions many embedded devices, and the court cases give you some idea about what you are allowed to do with the code for those devices.