2001 impala steering wheel controls

I have a 2001 Impala with the factory am/fm/cass/cd radio.  The radio and all it;s functions work fine except my steering wheel controls have stopped working.  The horn and cruise buttons all work.  I have checked all fuses and wires (I think) and everything looks good.  Where else should I be looking or checking?  Thanks

The problem could be many different things. Use a digital volt meter and set it for DC voltage. Install one probe in the dark blue wire in cavity A7 of the radio plug and the other probe in cavity A6 - Pink wire. With the radio turned on, try using the steering wheel controls. You should get a varied voltage depending on which control you select. If you don't, then you have a problem in either the wiring between the radio and the controls or the controls are defective. If you do get different voltages, then there is a problem with the radio.sparky

Thanks.   are you refering to the plug on the back of the radio itself?  Could I check it on the amp plug?  Trying to get to the back of the radio on this car is a real pain.