I am trying to connect up a non-Sony steering wheel remote (on a Chrysler vehicle) to the Sony stalk interface on a tuner and I need help!

The scenario:
On my Jeep the steering wheel remote switches are pulled to 5V by the car and pressing one of the six possible buttons causes the voltage level to drop to levels like 0.5v, 1.5v, 2v, 2.5v, 3v, 3.5v, 4.5v. Looking at the FAQ it appears the stalk interface on the tuner module has a 22K pull-up resistor going to 5V and the stalk is the just fed into an a/d on a PIC microprocessor. Normally with a Sony stalk attached it will have the same effect as above pressing different switches will cause a simple voltage divider and levels to be applied to the PIC. (BTW The 22K resistor inside the tuner is too high a value for the Jeep, if I disconnect the car feed and just use the 22K->5V pull-up I get levels voltage levels of less than 1v for all switches).

My reading of the interface is that it returns a packet containing the state of a shift button (that I am not using) plus the a/d level in the range of 0-0xff (for 0-5V). One packet being sent for a button down press and a second packet on button release. The latter always having a value of 0xff.

So far so good, I connect up the remote and press the buttons and then I hit the problem. Only two or three of the buttons are reported the rest just don't get 'seen'. Yet as far as I can tell they should. BTW I am using hijack to report stalk packets to the serial port.

After all of that these are the questions!
a) Does the pic just report a/d values or does it do any other processing.
b) What voltage range does the pic interpret i.e. is it covering the full 0-5V range or just a subset.
c) Any ideas where or what I am going wrong?