Have you checked the oil level in the power steering reservoir?
Have you checked all the fuses?
If the leak is near the front of the car it could be a split or loose hydraulic hose/pipe.
If it's just behind the front wheel it could be a leaking seal on the PS rack, look for a swollen or split gaiter.
The leak may be from somewhere else, and nothing to do with the steering
.
If the PS oil level is OK then the problem is probably down to a bad radio install. The original radio uses the speed signal to control the auto volume adjustment, if the radio is incompatable with this, or the fitter has hacked the wiring, the signal will be corrupted. As the PS is speed sensitive this can cause your problem.
So basically, check PS oil level, check fuses, if that's OK refit original radio/check radio wires for hacks. If that doesn't work it may be the PS pump has simply failed, not uncommon.
http://www.bba-reman.com/content.aspx?content=vauxhall_zafira_electric_hydraulic_power_steering_pump
paul888
September 2009