Seven people suffered minor injuries this morning in a fender bender between two MBTA trolley cars beneath an underpass near Cedar Grove Cemetery in Dorchester, an EMS official said.First responders initially had a tough time even finding the crash site given its location beneath a narrow bridge amid trees and tombstones, Boston Fire Department Deputy Chief Bart Shea said.“When we did finally locate it we determined it was a minor collision,” said Shea, who had no immediate information on how it happened.