STAMFORD — A city teenager’s failure to use a turn signal led to his arrest for allegedly stealing a car on the East Side Tuesday night. Capt. Richard Conklin said officers in the Narcotics and Organized Crime squad were patrolling on Custer Street at about 8:15 p.m. when they saw a 1999 Silver Audi A4 make an abrupt turn into the parking lot of 44 Custer Street. Because the driver did not signal the turn, the officers activated their lights and and pulled behind the vehicle.