already featured in gpxpatch 3.35 just rename cars.wad to i.e. cars_melborne2001.wad if you have a trackset for australia and go into gpxtrack.ini and change 0 to 1 in "enabletrackspecificcarswad"
This is really good news. The only drawback might be the size of files. If you have 17 trackspecific wad files and all are about 80MB, you end up with 1,360 GB of additional data in your Gp4 directory. Imagine you had a CART, a GT and a 1994 mod as well, you need a HD for GP4 alone
Plus you'd have to backup oyur game's directory on DVD.
check this thread (in Dutch) the tool does what you want. Selection of different cars.wads. I tested the first version and it works fine. You can have 2003, 2002 etc. season and a fantasy cars.wad as well as the GT cars. wad and a whole lot more if you want. You can even add your own preview pic of the carset. Very cool tool!
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say you wanted to have changes for malaysia like sauber with malaysian flag and minardi with malaysia for peace on it ,copy and paste your cars.wad and extract with gp4master,put in new files and create new wad cars_sepang2001.wad and put in gp4 directory.now in that same directory is gpxdata folder.open that up and you will see an ini.file called gpxtrack,open this up and you will see ENABLETRACKSPECIFICCARSWAD=0.change that 0 to 1 and now you will see the different liveries when using malaysian track.the other tracks will be using the default cars.wad