I actually just completed putting a '91 into my '90 Pioneer. [except fo 5 wires I have no clue what they are for. 3 on the dash side, 2 on the console side]
If you look at a '91, it's basically the same as earlier consoles, and has only compass and outside temp. I've found the only real difference is the swtiches, and how the light 'cage' is done. [one of my swtiches is screwed, but keep finding the older style is why I know]
I'd suspect that the '96, although has extra info that it can give in it's computer, it won't work due to the fact it needs extra sensors somewhere that it needs for that information. Unless you know what/where the sensors are, and can somehow mod them into your '91, they're pretty much useless at this point.
[If you somehow can get it mod'd, I suggest doing a write up on it, as I've yet to see one done for any overhead install. Working on doing one at present for what I've done with mine]
Good luck!!
