Problem finding blue tooth device after it has been removed from iPhone 5

My old car has no blue tooth capability so I purchased a FM blue tooth transmitter which plugs into the cigarette lighter socket and transmits through the car stereo. I paired it up and it has been working fine until recently when my son told my iPhone 5 to forget this device for the car kit and now my phone will not discover this device. I have check the instructions for the blue tooth device but it only says how to set it up the first time there are no instructions for a reset and I can not find a reset button on the device. Any advice?