If I remember my Jensens, don't they have fibre glass bodies? This means that your aerial will not have a very good ground plane - if at all. FM needs a decent aerial. I have two suggestions. Take a wire from the outside connector of the aerial's coax to a suitable earth (ground) point. (In the case of a fibre glass car look for earth wires near lights and connect to one of them. Be careful that the wire is earth and not 12V DC. Alternatively get a powered aerial that sticks to the windscreen (windshield), they don't need a ground plane.
Terry, April 2006