I have followed every direction to get these mods to work. I have GTA 5 on steam, using OpenIV with vanilla works installed, and ASI manager installed via OpenIV. I added the script line for the benatar and fmgt in the top left corner. Made sure the file name was benatar.yrf and fmgt.yrf as shown in the top right corner for the spawn name. I have a separate mods>update>x64>dlcpacks>benatar folder with the car mods name in the folder as the spawn name and the rpf file.
When installing simple trainer I also installed script hook V and scripthook dot.net
Simple trainer and menyoo both work great but will not spawn any mod vehicles of mine.
My Rig is more than capable to run these mods, Asus motherboard, Phenon 1090T CPU, Vengeance 32GB Ram DDR3, 6800 AMD graphics card.