I dont know if its still the case, but before steam path to install mods was like this ->
Automation main folder -> assets -> meshes -> there you have 2 folders, car parts and cars.
Car parts - fixtures, such as exhausts, headlights etc.
Cars - car bodies.
If foler in your mod archive starts with assets, just drag it into main Automation directory.
If you have just mod inside such as 00van, 70pickup or similar, take that folder and place it whenever:
a)If its a car - go to Automation main folder -> assets -> meshes -> cars -> and now depends what car do you have, small, big, front, mid etc. Hardly matter, just put it into place where you see few other folders with names and number like 00hatch, etc.
b) If its a fixture - go to Automation main folder -> assets -> meshes - car parts -> and put content of archive into what you downloaded - grilles to grilles, exhaust to exhaust etc.
THO i have no idea if it still works, as iv converted to steam on day 1 and have easy life with workshop. 
Though it probably still work, because im using badge in car parts directory, because it’s not available on workshop.
If you still need help, dont understand what i wrote, il make new post with screenshots to make it easier. 