Had the same problem in my 03 - when it was under warranty I tried to get the stealer to fix the problem - but they never did for very long. I then decided to take the radio out myself and have a look. There's a connector for the steering wheel controls which merges into the main harness and I discovered a couple of pins were not seated in the housing very well - guess which ones! Now worked for a year with no problems 
My advice is to take out the radio and have a close look at the connectors
Andy
2003 DIIa TD5
Oval Split level roof rack
DIY Storage system
Barrett HF - Tango 1026
GME TX3200
Traxide dual battery controller
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