Do the trucks that are doing this have rear combination lights(brake,Backup, turn)? I did have this problem once with one of these it was several years ago and if I remember right it was either in the wiring going up thru or by the OHG Rear pillar post or it was in the light assembly itself....if it has brake lights check to see if one is dimmer than the other....I believe it was in the light assembly and was shorted somehow to the Backup light bulb....even if it dosent have the lights in the back the wiring will be there along with the plugs and connections....look for rubbing and chafing in the harness..the back up alarm is feed by the same wire that operates the backup lights