Snus

They say to put it in the latest patchday.  The game reads the vehicles installed from the latest patchday to the oldest.  Oldest being patchday1 and the latest being patchday23 I think.  I think there is a police2 or police3 in a patchday1, patchday3 or patchday4 for example.  The game is going to pull the vehicle from, for example, patchday4 instead of patchday1 because patchday4 is the latest.  So if there there is a police2 in patchdays 1 and 4, the game will take the vehicle from patchday4 since it is the latest.  If you put the police2 in the latest patchday23 then it will pull the police vehicle (police2 for example)  from patchday23 instead of patchday4 or patchday1.  Hope you followed all that.  Yeah, it is confusing.  I have had a problem with the first police vehicle where it would not show up in game unless i put it in the x64e.rpf file.  I had to copy the x64e.rpf to the mods folder because you don't want to make changes to those types of files in the Main GTAV folder.  The full path is x64e.rpf/levels/gta5/vehicles.rpf.  There you can drop your replacement vehicles in there if they don't show up in game when you drop them into the patchdays folders.  At least that is what I had to do to the Police,Taxi,Train,Bus vehicles when I replaced them.  I don't know why, but that worked for me. 
 
If you are doing ADD-ON vehicles, that is a lot more complicated.  I suggest using Lenny's Mod Loader (LML) or Albo's Modding DLC.  I use Albo's because I haven't tried Lenny's yet. 
HAHA, I just noticed after typing all that, this is an old post.  Well, there you go.  😆