i want to make grunts drive ghost. but grunts can't go to the driver seat of ghost.
what shall i do?

the grunts do not have the animations to get into the driver's seat of the ghost.
this means that even is you script them to get in, they wouldn't.

think of it like a toddler being put in the driver's seat and being told to drive.
the toddler wouldn't know how to put their hands and hold on to the steering wheel, nor would he/she know how to put their feet on the accelerator pedal or the brakes.
(halo just doesn't want grunts to have accidents, lol)

there are, however, two ways to fix this problem.
1. make the grunt use animations that it already has so that it can use the vehicle.
(the grunt would look odd though, like if you give the grunt the covie gun turret animations when using the ghost............you know what i mean)
2. create ghost animations for the grunt to use when entering the ghost (that includes the entering, driving and exiting animations etc)

Change the seat label from g-driver to GT-gunner
It will look like this ingame http://screenshot.xfire.com/screenshot/natural/1f31cf3d13621042286c946f9410d59704b3ae76.png

Lol I just noticed that, I only read the bigger post's.

It'll look weird when the player gets in. There's a more complicated way, but if you get it, it's the best method.

1) Open the grunt's model_animations tag
2) Under 'Units', select "GT-gunner"
3) Click "Duplicate"
4) Rename the duplicate to "G-driver"
5) Save

the grunts do not have the animations to get into the driver's seat of the ghost.
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I got to know it in the past, but i don't know how to fix it.

By the way, how to make animations?
i downloaded gmax , but it can't edit gbxmodels.
what shall i do?

Did you ignore all the posts above you? My method gives the grunts animations to enter the ghost, but it won't exactly look the best. And to edit gbxmodels, you have to use the gbxmodel importer, import it in, modify it, export as a JMS, compile the JMS in tool.

http://hce.halomaps.org/index.cfm?fid=3358

Just use model the model animations tag from here. They can drive every vehicle.