Since I have had the car the steering wheel controls have not worked too well. The volume control seemed to switch mode instead! Apparently after some reading this is usually because the contacts get dirty and cause resistance in the circuit thus reducing the voltage. The signals from the switch to the radio are what they call analogue which means that the radio reacts to different voltage levels being sent down the wires from the switch. Which explains how all the different functions the 3 knobs do in the switch only go down 2 wires to the radio. This is due to the different voltages from the different resisters in side the switch to produce the different voltage levels for each signal to control the different radio functions. If dirt builds up on the contacts this can effect the resistance and hence the signal votage and then confusing the radio functions.
Anyway I bought another switch off eBay gave it a good clean with contact cleaner spray to replace the original.
Disconnected the battery (make sure red light is not on on the sat nav box first), removed 2 torx screws on the back of the steering wheel, lifted the centre section, unclipped the air bag wire to expose all the wiring to the switch.
Unscrewed and unplugged the original switches and replaced them and reassembled.
Before I fitted the replacement switches I cleaned the horn buttons and repainted the little horn impressions on them to look new.
MG ZT info mods and maintenance 