detMaxPayne

All you need to do is use the latest patchday, which will override all the previous patchdays. That way, you have no erroneous searching.
The latest patchday currently is: GTAV/mods/update/x64/dlcpacks/patchday28g9ecng/dlc.rpf/x64/levels/gta5/vehicles.rpf/

there is 2 ways you could do this
1) for the car you want to replace, do a search, lets take sheriff for example, do a search "in game files only" and see how many pops up

as you can see, 3 spots for it show up (im showing 2 in the pic) and if you look at the folder names, one says patchday1ng and one says patchday3ng
patchday3ng is a higher number then 1 so it must of came out later at some point, and the game will load that after patchday1 so anything in there will replace what patchday1 is replacing, so your best bet is to place your sheriff replacement car in patchday3ng
 
2) the 2nd way is a lot more tricky, but it will save you from having to search for the car you want to replace
what i did was download a addon car, and all my cars go in there

now the tricky part is , you have to find the meta file and delete the lines for it from the update.rpf , as it will load from there and not your new dlc
i made a txt file reminding me where and what to delete so that it loads my versions correctly
assuming you put your version of the car (any edits that the readme says to do of the car in the correct meta file of the addon car (vehicle.meta, carcols.meta handling.meta exc)