Surveillance cameras are monitored by police officers at the police command center in the border city of Tijuana in this April 17, 2007 file photo. Endemic police corruption -- ranging from traffic violation bribes to openly aiding drug gangs -- is undermining President Felipe Calderon's attempt to crush powerful cartels with thousands of troops and federal police. Informal alliances between corrupt police and narco gangs are frustrating soldiers who set up road blocks, scour towns and search houses across Mexico for drugs and guns under Calderon's military drive, which began December 2006. To match feature MEXICO-DRUGS/POLICE