How was this done?
Glad you asked - (nearly) every model in the game has numerous markers on it.
Markers are simple a 0 dimensional point in a 3d area around (or inside) the
model.
There is a marker for every point needed - a marker to show where your
player grabs onto a gun, where the bullets come out of a gun, where the
turret is on a warthog, where you get into a warthog, etc.
I found there are often also markers for the FPP view on many of the vehicle
models, created by Bungie, even though in Halo 2 retail they are never used.
Due to this fact, they were also rather lazy in enabling them. The passenger
seat of the warthog was already set up to use it's corresponding FPP marker.
But the driver seat wasn't, and the banshee is set up referencing a
non-existent marker.
So all I did was put in the names of the correspoing SIDs in the seats
reflexive in the .vehi files. Compare my modded map and an original map for
more info.