Are you having problems installing vehicles?
Does it not work when you replace the files?
Simply use this: Vehicle Mod Installer v1.3
Step One: Download the desired vehicle.
Step Two: Open the Installer and find your GTA directory in the top browse box.
Step Three: Browse to the main folder of the vehicle in the bottom box.
Step Four: Click the 'Next' button in the bottom right
Step Five: Check all files are correct. (NOTE: The installer detects ReadMe.txts for handling.dat etc!)
Step Six: Click Next
Step Seven: Select the car you want to replace. If you want fill in the top box to rename the car so intead of it being called "Police Cruiser" it will be called "CVPI" in-game when you enter the vehicle!
Step Eight: Click install follow on-screen instructions if any appear.
Step Nine: Launch GTA IV and use your car!
Did it help? Use the comments to let me know!