Car Interface

I am working on making a steering wheel interface. I want to interface my existing controls to my aftermarket head unit. I know they make aftermarket units that do this but you are only allowed 1 function per button. I want 2 functions per button. If you just do on/off would be one function or hold for x sec another function. Then the head unit needs to see a certain voltage on its remote input to control.
Simplified
1 Read voltage from steering wheel controls (2 pins needed) Buttons on steering wheel
2 Based on the voltage control digital pot to supply voltage to radio

Hardware:
Arduino Pro Mini 5v
Digtial Pot AD5206

Here are resistances for the buttons on the steering wheel.
Seek (down) Depress button. 165
Volume (down) Depress VOL down button. 652
Mode/end Depress button. 0
Seek (up) Depress button. 165
Volume (up) Depress VOL up button. 652
Phone/send Depress button. 0