This seems to not be working for a lot of people. So attempt at your discretion.
First, I would like to thank XBLToothPik because without his posts I would have no idea this was possible.
Second, this works for me and has been successful numerous times so if it doesn't work for you sorry.
For this tutorial I am using the buzzard but you can use whatever vehicle you want.
So you are going to need to know how to get your common.rpf file. If you don't there are plenty of threads here on this forum to inform you.

From your common you are going to need to extract your vehicles.meta
[ Register or Signin to view external links. ]

Next you will open the file in notepad and search for "taxi" and replace the modelName, txdName, and gameName to reflect the vehicle you are using like I did below.
[ Register or Signin to view external links. ]

Now scroll down the page a little and look for the section that says "<flags>"
[ Register or Signin to view external links. ]


and delete "FLAG_NO_RESPRAY"
[ Register or Signin to view external links. ]



Now save the file and close it.

Open Liberty and import the vehicles.meta then save and quit.
[ Register or Signin to view external links. ]

Now replace the edited common.rpf into your iso and run through abgx to make sure the stealth has been verified and load your modded iso in whichever way you want and have fun. Remember though if it is anything other than a car or truck it is going to probably drive odd as hell since it is not the correct model for the taxi's handling.
[ Register or Signin to view external links. ]
[ Register or Signin to view external links. ] Sans