( source - dabonwheels.co.uk )
It's easy to damage equipment if you don't know what you are doing so, if you are the least bit uncertain, dabonwheels recommend that you arrange for a professional to carry out this work.
Please note that results with built-in-the-glass aerials and 'Sharkfin' aerials are likely to be very poor, i.e. not worthwhile.
1) Identify the FM/AM amplifier.
2) Disconnect and insulate the power supply (if any) to the amplifier.
3) Identify the input to the amplifier from the aerial element. Carefully disconnect.
4) Identify the ouput from the amplifier to the radio. Carefully disconnect.
5) Carefully connect the cable that you disconnected from amplifier input to the cable that you
disconnected from the amplifier output. If the cable(s) need to be extended you should use screened
Your aerial is now converted to passive.
now thats all very well... if you know which wires are which