Weak Signal on FM and no AM Signal After Changing Aeriel

I recently changed the base and aeriel of my fabia I to an amplified base. The last one was not. I have I took a switched Positive to the base for power and presume the Amp gets power from the Car Chassis. I just done a straight swap and the signal is still very weak on FM and none on AM. Have you got to earth the aeriel at the base aswell. I.e. Get a serated washer so it cuts through the paint. I thought the Earth from the Coxial cable would be enough to power it. Any Ideas? No instrunctions came with the aeriel