This is the tutorial I followed exactly(to my knowledge.)
There are a couple of concerns that might of made things go wrong: I am pretty sure that his police cruiser is not the default one. When I was changing the textures in the .ytd. files, I replaced policenew_sign_1 w/ the skin provided, then delete the rest of the policenew_signs that were there previously. Then, I go to Police.yft and find that it didn't work; either the skin is on the vehicle but like a kid threw cans of paint all over the car, or the whole vehicle texture is on the roof of the car.
Note: I also prefer the sheriff vehicle goes into the Sheriff spot.
If this doesn't indicate any truth and you don't know what I am doing wrong, I will gladly go through my process more thoroughly.