The ariel on the roof is an amplified ariel, its built into the base of it and this can fail over time when water etc gets into it. However if someones buggered up the cable for you i would make sure that its spot on before you go down the more expensive routes of new ariels etc
 
Er the Aerial is a co-ax cable, requires soldering. if its not done "propper" you will get crap reception.

needs to be 100% spot on!

and also it's wrapped seperately to the main stereo power/speaker loom

from the pic it looks like it's still in the back