if you have a failed bulb the current from the taillamps can backfeed through the signal lamp circuit causing the indicator lamp to illuminate. the bulbs are all grounded through the taillamp housings by the metal tab on the bulb socket, the bulbs will not work properly without all of them being installed in their holes and the lamp housings being grounded. other than through ground there is no connection between the taillamps and the signal/brake lamps,they are seperate circuits. hope this helps