You need to change the names of the ytd, and both yft file's (keeping the _hi on the one yft)
You also need to edit the carvaiations and Vehicles meta for these file to reflect the new model name Police2 For example,
This Vehicle meta needs these lines in bold changed.
It will look something like this.
You may notice the line in red. This line is a case by case basis line, for example, if it was a suv, it would be kinda weird if it uses a cars handling, so change this as needed, its best to set it to what ever it was before, Most the cars are running "police" as their handling.
The Line in Green is the sound file. for the most part you shouldn't have to change this unless you have no sound, or the wrong sound. If you do just convert the line like I have and put whichever sound model you want.
Bear in mind, that this is the most common and easiest file to screw up, and if you do. Your game WILL CRASH. No ifs and's or buts. It WILL CRASH. Back up whne ever you edit these files.
now to carvariations. This one can still cause a crash, luckily, its a lot simpler.
Changes To
Boom Done. If the car has carcol's files use its carcol's and just change the model name line to reflect the slot its in.
As for ELS files, jsut rename the .xml file to match the model name. (police.xml > police2.xml)
This is also how you add addon cars, by creating these same files. But thats another topic.
Also (I assume your new to gta modding) if you add to many cars, you can get a game crash because of a gameconfig setting. To test this, when you get a crash and you've checked all your files and they all make sense, Try removing a car that you know to be working. If the game then loads with your new car, but crashes when you put the removed one back in, that is a config setting causing it. (I fought with this for weeks trying to figure out why. ;P)