The thing i would wonder about is whether in fact each button is actually connected to a discreet pair of wires. If those buttons illuminate then you probably have a separate power and ground for that, which would leave even fewer wires per switch than you have now. This makes me think that it is like most cars which use a voltage divider network to indicate a different button press over a single wire.
What I would try is go to pac audio's website and look for information on a unit called the SWI-X. See if your car is listed there. If it is, it will probably tell you something to the effect that that white wire is actually carrying the signal for all the buttons... i.e. it is the ground for the voltage divider network rather than simply a common ground for each button.
If that's the case then you're in the same boat as I am and you need to employ an A/D converter to interpret the voltage on that signal line and translate it into discreet switch closures that the Ipac can understand.
One reason I think that this is the case is that as you say, four switches all activate the same trigger. This would suggest that whenever you close a switch, you're closing between the supply (maybe the green wire) and the ground (the white wire). The fact that there is a small (but different) resistor on each switch makes no difference to the ipac, and it just sees it as the same button being pressed each time.
I don't know why they would use two divider networks instead of one, but if they needed to space out the resistances for reliability, then that would make sense... because if the A/D converter that is reading those switches doesn't have a high enough resolution, or as i suspect, temperature can make resistance vary too widely to have 8 switches on a single line.
okay... last thing i have for you: check this page: http://www.pac-audio.com/bulletins/swicalculator.htm. It should give you an idea of how this type of network works and also explain the results you're getting.