This is something i've always wanted to be able to make for SA, but until recently I didn't know how to give a vehicle 8 wheels or how to make the front 4 wheels turn. Using a technique by Pinky (at gtaforums.com), I was able to trick the game into basically re-using dummies for mutliple wheels. See my forum thread for a more technical description.
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However there are a couple major issues that keep this from being a practical or finished mod. Because of the rearrangement of things in the DFF, the game now spawns the truck upside down! In order to use this, you have to get in a helicopter or use the jetpack to hover over the edge of a building or other ledge and spawn it in a way so that it when it falls, it clips the edge of the building and flips over. This is a problem that absolutly needs to be fixed before a final release is made.
The other problem isn't quite as serious, but the steering is opposite due to a combination of the changes that were made to the dummies. Left is right and right is left. This is easy to counteract by just temporarly switching the controls around, but that is sort of a hassle.
I'm releasing this as a beta due to these 2 major problems, but am still working on it and would love any help that anybody may be able to give in solving one or either of these problems.
If you understand the problems but still want to try this vehicle (I suggest you do :)), click here to download it.