you mean like....
...Rockstar GamesGTA San Andreasmodelsvehicle.txd / genericvehicleshatter128?
or you mean like the ones who came with the vehicles by default(like in the vehiclename.txd file)?
for txd files simple edit(import/export) you may like to use TXDWorkshop
btw, for GT:SA what i use the most is ZModeler.