This will work on all Pioneer headunits that have a wired remote port on the back.
This is a 3,5mm stereo jack with "W/R" written above it.

In most cases this plug is black. The blue one is an aux, line in connector. DO NOT USE THE BLUE CONNECTOR!!!!
I'm not responsible if this mod destroys your headunit. All I know is that it works on my Pioneer 7900UB.

I'm happy to answer all your questions:

This is the back of my 7900UB:
Here are all 10 functions, I don't think there are more functions.
Each function has it's own resistant value. All values are in Kilo Ω (kΩ).
You have a wide choise of resistors, I've listed them all:
You only need 3 wires to connect all switches!
Here is the working scematic: