Your dealer and the Honda rep sound completely clueless about electrical systems. Unless you damaged the wiring or blew a fuse somehow, no accessory you added, assuming said accessory is still working properly, could possibly fail the tail lights. Tail light failure has been a fairly common problem on the '21 P1000, as reported here be several owners. It has been covered by warranty and appears to have been a bad batch - though Honda has just replaced them and admitted to nothing. Do a search and print out to show the dealer you are not alone.