or:http://forums.halomods.com/viewtopic.php?t=59268
(You dont need to do that with sparkedit)
1.then open halo map tools
2.then go down to [vehi] and then klick "scorpion" and copy the meta code
3.when you have copy the meta code go to [proj]
4.klick plasma pistol\bolt and replace the meta code with scorpion meta code
To make you dont die when you shoot:
5.now go to [weap] and go down to plasma pistol
6.scroll down to triggers and select 0 in the drop down box
7.and scroll down to "X spawn Coord" and set 50 in the box
8.close map file and play halo
9.Done
edit:i mean press dont klick

.:Updated:.
How to get them shoot at you!
1.open HMT and open HMT again
2.press open in one of the HMT and open up b40
3.and the other HMT bloodgulch
4.go to actor (both of them) and make them same as marines actor
5.Done