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.