PIC's are kinda complicated to use unless you're already fairly solid on computer programming and electronics. (If you are then they're dead simple! But those are pretty high prerequisites for some.)
I'm going to get around to making a whole writeup on the PIC circuit someday... In a nutshell, it can read an analog voltage on some of its input pins. A simple input circuit generates a particular voltage based on which button is pressed, which the PIC can interpret... So now it knows which button is pressed. Then the output pins can do whatever you want.
I have mine set up to mimic a wired remote to my stereo head unit for volume. But you could also hard-wire it to a little USB keyboard to generate "key presses" to the computer -- for instance the + and - keys, or even the VOL+ and VOL- keys on an extended keyboard with multi-media buttons -- and then that could control the computer's volume.
I'm thinking you could also do all of this with a Fusion Brain, or even an off-the-shelf steering wheel control adapter like the PAC SWI-X Steering Wheel Audio Interface. That one just works by sending IR remote commands, so you'd have to get an IR input for your computer, but those are fairly cheap.
Sorry to not answer your question about the PIC circuit. I really do think it's a cool way to go, but it'll have to be a really long involved writeup in order to be very helpful.
Bump man, been a while, how are things looking?
Wow, it has been awhile. Thanks for asking. Truth be told, I've been spending more time on my music than my car PC. (The music's going great though, by the way.)
So... Designing this was way fun. Building it was way fun. Showing it off is way fun. But uh, looking at the long list of system bugs to fix and fabrication things to finish... well... I'm starting to have wishful thoughts about an off-the-shelf navigation deck that just *works* and doesn't clutter my trunk. Thinking this might turn into a nice media PC for the living room.
So sadly, looks like another one bites the dust. Haven't ripped anything out yet, but I just don't think I'll find the time to finish and maintain it. All is not lost though. Still have a great PC to use elsewhere, and still have a car wired with good speakers and good power distribution. Just 'cuz it won't have a PC doesn't mean I'm not gonna do it up nice. :)
I'll post more pictures when it takes shape.
Sorry to hear the issues are holding you back from keeping the CarPC.
I was looking forward to seeing what you were doing.
I tend to be the same way about getting taken off in other directions. Add to that a full time job, wife, kids, Full time School and it's hard to find time for everything. This is why I'm trying to keep my system as simple as possible, while still being usable, and functional.
I'll leave the stock head unit in, and just plug a stereo cable into the aux input for it to access my music. The under the seat mount seems to be the easiest. I may even just use an inverter to power it.