For this glitch to work you need either a carbine or a battle rifle.
Get to an anti air wraith and melee the cockpit door (located beneath turret) until it comes off.
Melee the drver in the head so his armor comes off, start crouching in the cockpit area until it doesnt say "press RB to plant grenade".
When the spot is found shoot the driver in the head while pressing RB and you will be able to use it to full extent.
Note: when you get out it will explode because it's not supposed to be operational, also someone else can use the turret