This is an updated and completed version of SuperFranky22's mod, which enhances ikt's Add-On Vehicle Spawner mod (version 1.4.2).

I've included most of the original images he included, though I replaced many and added all current DLC vehicles as well.

This is current for version 1.0.1868.1 of Grand Theft Auto V.

Requirements:

GTA V v1.0.877.1 or later
ScriptHookV
Add-On Vehicle Spawner 1.4.2 by ikt

Installation:

Replace the img folder in \Grand Theft Auto V\AddonSpawner\ or put the image files directly in that folder yourself.

The Add-On Vehicle Spawner mod supports your own custom add-on vehicles (in addition to stock vehicles). Since I don't know what vehicles you're adding, obviously I haven't included images for those.

However, if you want to add your own the process is simple. Screenshot the vehicle in-game, resize that image to 300 or less kb, and re-name the image to the spawn name of the vehicle. Then place that image into the same img folder. Full instructions are included in the readme for the original mod.