okay so i'll just go through how to install this mod, as an example: http://www.gtagaming.com/downloads/g...icle-mods/3062
(you can use WinRAR
to extract it)
After downloading the mod, get SparkIV
. Open up SparkIV, and click Browse. Find the path pc/models/cdimages in your GTA IV folder and double click on "vehicles.img" to edit it. A new window will pop up and you'll see a bunch of .wft and .wtd files for all the cars. At the top, click the Import button, and browse to the police2.wtd and police2.wft files you downloaded and extracted for the NYPD mod. Import those and then click the Rebuild button. After a while you'll get a confirmation message, then close SparkIV and you'll be good to go!
Let me know if you'd like me to elaborate on anything!