Some radio signals aren't really supposed to be directionally biased but they are because of "local conditions" which includes their being mounted in a car. But to suggest an easy "fix", have you changed the batteries in the transmitter? Sometimes weak batteries may lead to such a weak signal that things don't work right? You may have to reprogram the transmitter after you change the batteries, like you do on a "one unit does all channel changer for your TV". The channel changer is optical, but it works about the same for programming.