before you do any thing else try the outside antenna and see if it works better.
strip about 3" of insulation off the end of the old white wire antenna and wrap the stripped wire around the pin sticking out of the roof antenna cable and tape it there.
plug the old antenna wire into the radio and see if it works better.
if it does i would shorten up the old antenna wire strip the end and connect it to the antenna wire pin again.
i would use the excess wire to strip and make a connection from the ground part of the roof antenna cable to the metal case of the radio and see if that makes it better.
you will not find a ready made adapter for it but you could make your own by buying a female motorola jack and soldering the old antenna wire to it.
