This is just some things people should know about car mods that a lot of people don't realize and some things I find myself having to answer questions about repeatedly.
Now obviously the simple stuff is that car mods are installed with openIV, pretty much everyone knows that but if you are new to the modding scene here you go.
Download OpenIV here
You can find the cars to replace in different locations inside "vehicle.rpf" files, these files are located inside your gta folders including "x64e" "x64w" and "update" folders, cars are sometimes duplicated, if a car was in the game when it launched it will be in the "x64e" vehicles.rpf, however if the car got updated in a dlc or patch, the car will ALSO be in the "update" or "x64w" dlc and patch vehicle.rpf's
Make sure you hit "edit mode" at the top of openIV before you try doing anything
An easy way to find the newest file for a car is to click "Tools" then "search" on openIV (Hotkeys Ctrl + F3) then type in the name of the car you are replacing,
For example if you search adder_hi it will bring up every file in the entire game with that name, that particular car has never been updated so it will show you the one file by that name located in x64e.rpf>levels>gta5>vehicles.rpf
Click the file to be taken to that location where you can then replace it
(Note i have two adder_hi folders in my GTA in this screenshot because i'm using a mods folder, i highly advise you do this, OpenIV will do it for you if you let it when it asks you, it duplicates your car files so if something goes wrong you can revert to the original)
If you search for a car that has multiple versions, look at the names of the folders, for example if it shows a copy of the car in the x64e.rpf location, and in the patchday2ng location, then you know to replace the patchday2ng car file as that is the newer location that the game is using
A car mod can replace ANY car in the game regardless of type,
You do NOT need to install it over the specific car it has been named to replace, there are only two things that will prevent it from working correctly if you replace a different car.
Reason 1: if the car uses any GTA V stock textures, if it a vanilla car edit then it will use them, if you place one of these cars in the wrong location and it doesn't have all the necessary stock textures already in its .ytd file, then it will be unable to locate them and it's interior textures etc will show up as a mess ingame, the textures it wants should be in the vehshare file so if you can dig them out and put them in the .ytd it should solve any issues, example cars, this schafter is based off the stock GTA V car, (https://www.gta5-mods.com/vehicles/schafter-shooting-brake) it uses interior textures and dial textures that the original car uses so it must replace an ingame shafter for the textures to look pretty, if you don't care about a few missing textures you can still go ahead and install over a different car. This Koenigsegg however has no such texture restrictions, https://www.gta5-mods.com/vehicles/2015-koenigsegg-regera-convertible you could install that over ANY car without any texture issues as everything it needs is included in the .ytd file.
Reason 2: if your modded car cant behave well on the handling of whatever car you replaced this can be a problem, most cars will work on anything from the adder to the panto, but some cars will flip over when you turn or the suspension travel will be too much and the wheels might scrape or go through the wheel arches, these can be fixed by changing the cars info lines the handling.meta file, take the handling info from the car it was supposed to replace, and put it over the handling of the car you have replaced.
You can still install an add on car as a replace car
All you have to do is put the dlc.rpf of the add on somewhere (anywhere) in your gta folder then use openIV to open it, extract the car files, then use them to replace whatever car you want.
Modded car .yft files are identicle a lot of the time,
When most people make a car the .yft file is just copy and renamed so there is (example) adder.yft and adder_hi.yft, it is mainly stock GTA cars that have two different .yft files, some .yfts are slightly different though, but its good that you know you don't need two seperate ones for the mod to work so that if you only have one .yft you know you can still install it just use the same .yft to replace both .yfts in openiv.
You do not need to rename a file to replace a different car.
Simply right click on the car you wish to replace in OpenIV, then select replace from the menu that appears, then select the car file you wish (regardless of name) it will automatically name to the car you are replacing.
You can make any car add on quite easily by using a different cars add on
If your favorite car mod doesn't have add on simply download an add on car that you don't want, there are tons of add on cars it isn't difficult to find one you don't want, install it correctly, then open the dlc.rpf file that contains the car model, replace the files with the car you want, and viola the car you wanted will now work as add on, replace the handling lines in the file (also located in the dlc.rpf file) and you can make it handle how you want.
A police car does not have to replace a police car and a civilian car can replace a police car
If you replace a civilian car with a police car, the sirens will not work, however the car will work fine aside from those sirens.
if you replace a police car with a civilian car, the car will be able to turn on a siren have a police horn and its normal taillights etc should flash on and off like an undercover police car
Police cars can be made add on too
Use this pack as a guide for the needed handling.meta and vehicles.meta stuff https://www.gta5-mods.com/vehicles/more-police-cars-to-add-v1-2
When a car has extra's these are parts that spawn randomly on the car when it appears ingame, or can be manually toggled using a trainer
Vanilla GTA V cars usually have things like coffee cups or ashtrays as extra parts, modded cars usually have things like hardtop roofs bullbars etc
If a car has tuning parts you don't have to install the tuning parts if you cant be bothered.
The car will still work fine it just wont have the special customization parts
If you want to install a skin for a car but you downloaded it and its just a .dds picture file and you're confused as to what to do with it, what you do is go to openIV, go to the .ytd file of the car you want to apply the skin to, for example cheetah.ytd, make sure you are in edit mode on openiv, then double click to open the .ytd file, then either use the import button to bring the texture in, or find the current texture, click it, then hit replace
That's all that's come to my mind currently, I have some work to do so leaving it at that, will update with more info if I think of anything, any questions post them below and i'll do my best to answer.