Working with Gp4master.
1) Create a temporary folder.
2) Launch Gp4master and extract cars.wad to that temp folder.
3) Look at the directory structure for cars.wad and note where you new files should be placed (eg, carshape files can be seen to be in Cars\ , so new car files will go there)
4) prepare your new files. Most of them will be tex files. If you have tga, load them into say csc4 and save as tex. Save them somewhere handy.
5) Copy your new files into the unwadded cars.wad in your temp folder, overwriting files as you go. Remember where you determined they should be placed (3)
6) Go into Gp4master again and select create wad. Tell it the folder, with full path, to make a new wad from. That will be the temp folder created in (1). Do not call you new wad cars.wad!! Gp4master places the new wad the most bizarre places and you might do damage if the new wad has a filename in use, as it overwrites without prompting you. Call it newcars.wad or something.
7) Rename your old cars.wad to a backup-filename.
8) Search for your "newcars.wad", or whatever you called it. Once found rename to cars.wad and place in your Gp4 folder.
9) All done.
It's only after we've lost everything, that we are free to do anything.