The thing that baffles me is that Steven was convicted on the basis that the murder was in the garage and brendan was convicted on his confession which states the murder happened in the trailer.
How can two people be found guilty of the same crime in two different crime scenes? Surely one scenario at least cannot be true.
Brendan says that they stabbed her in the trailer and cut her throat, but she wasn't dead. The detectives then led the story to the garage where the spend casing was found.
Brendan initially says it happened outside but was then persuaded that it happened inside which puts the final bullet in the garage near the casing.