Audi a4 cabriolet 2004 car the cd players works fine but not the radio. I took the plug of at the back of the unit and found out: 1.if you put just the centre wire of the antenna radio cable in to the unit it works. but 2. if you touch the outer cable to the outside of where the outer cable goes the signal is lost. Why could this be??

You have what is called a short in the antenna. mosy likely you will need to replace the antenna and reroute it.
Someone may have - just speculation but they may have had the dash out to replace say a heater core and then they routed the antenna wire and shorted it out.
You might be better off just taking it to a stereo shop and paying 10 bucks for their opinion.
Take the radio out to where the tech can get to it and let him look it over - then put it back your self. A Humble request and a 10.00 tip can go a long way here.

print this out and show the tech - he might give his eyes on advice free.
Best of it