I saw someone comment on a Firefight video earlier saying you could drive a Wraith in ODST using the same method that people used in Halo 3 to get in anti aircraft Wraiths. I finally managed to do it on Lost Platoon after discovering that you needed to use a carbine and not a pistol to kill the driver.
I've put a video showing how to do it in my File Share and I'll say some notes here.
Bungie.net : Halo 3 File Details
The gunner must be dead.
You need to stand in a position that doesn't say Hold B/RB to enter, the place on the video worked every time for me.
You have to disable the vehicle with a plasma shot but you don't have long after to get in.
Hold the enter/reload button while you kill the driver. As soon as he dies you will climb in.
You
must use a carbine to headshot the driver, the pistol doesn't work.
You can get a friend in the turret but make sure they leave first when departing as the Wraith will explode when the driver leaves!
See what high scores you can get now

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