Well, do you still have the antenna lead somwhere in the trunk? If you do, reconnect the ground of the antenna lead to any body part. That should help a little. If you only listen to FM, you should be O.K. For good AM you will need an antena. FM will pass through almost anything, but AM needs help since it is a much lower wavelenth (but much higher bandwidth so it can go for miles outside of any FM station).