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.
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

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