Did you go back into your update.rpf and re-add them to your dlclist and extratitleupdatedata xml files?

@Akila_Reigns Wow! I had absolutely no idea.... I'm gonna try this out later. Thanks !

You can either choose to add OR replace existing cars, depending on whether or not the ones you are looking to get actually give you both options. Adding cars is just like how it sounds. It adds a new car, without replacing an existing vanilla one. Replacing a car, replaces the existing vanilla car with whatever the modded car is. As for the NPC question, I am assuming you are asking if NPC's would be seen driving the modded cars around. I am pretty sure the answer is no, if you add the car. If you replace a car, there is a chance that NPC's can be seen driving around a modded car. You spawn the added cars in via a trainer mod. I am not sure if there is any other way to get them besides that. Hope that cleared some things up for you

Try using this:
https://www.gta5-mods.com/misc/gameconfig-xml-add-more-addons