Former NFL head coach Dennis Green joins Westwood One's NFL broadcast crew this year as an analyst for the Network's Thursday Night Football coverage.   Green compiled a regular season record of 97-62 as the head coach of the Arizona Cardinals and Minnesota Vikings.   His best season was in 1998, when the Vikings finished 15-1 before losing in the NFC Championship Game.   Green also served as an assistant coach for the San Francisco 49ers for four seasons, earning a Super Bowl ring in 1988-89.   He was also successful at the college level, holding coaching positions with and helping turn around programs at the University of Iowa, University of Dayton, Northwestern University and Stanford University.   When he accepted the head coaching position at Northwestern University, Green became just the second African-American head coach in NCAA Division I-A history, an achievement he later duplicated in the NFL when he joined the Vikings.