I plan to do this with my TJ some time this year...
I have a 2001 so the wheel is different so the first step for me is to confirm that I can put the new wheel into the old Jeep. If the splines are different I may need to go further downstream on the parts list and change the column.
The next step is the clock spring from the Grand Cherokee will need to be fitted to have enough wires available for the Cruise + Audio Controls.
Somewhere in the middle of the project I'll try to fit 2003 Cruise Control.
The next step is using a universal interface to interperet the signals coming out of the steering wheel. The one I have chosen is programmable and works on Infra-Red. You mount a transmittor up on the roll cage aimed at the aftermarket head unit and it pretends to be the headunits remote control.
Therefore, ANYTHING that was controlled by IR you can now do.
So I intend to have two, maybe three universal adapters with relays and a toggle switch to select which one gets the signals.
Mode 1 = Audio Controls (my Alpine)
Mode 2 = Navigation Controls (my Garmin)
Mode 3 = Maybe find a way of integrating IR control with my Carputer, or use to control my Satellite Radio
Making it all work with a factory audio system I have not considered... I threw my factory head unit out a LONG time ago