PICS BELOW !!
You gain the hood ornaments by defeating the 13 Scrotus convoys scattered in the desert.
In order to unlock a convoy you have to drive on the green path showed on the maps i posted below and follow the road (usually the roads have blood or tire signs) , then continue your driving on the marked path until you will see a dust cloud rising from the road more forward from you: it's the convoy ! Reach it and start dealing damage to all of them (usually they are about six or seven enemy vehicles)
Remember that even if you almost destroyed the convoy . if you lost the eye contact with them (in example for repair the car) you have to start again . So i suggest you to have an excellent armour on your car or at least to have the perk of 100% less repairing time (Chum's tools powerup) . Also your attack factor is important, so reach the convoy with a lot rifle ammo, a good level harpoon and a lot of stick of thunder (the RPG look alike weapon)
All the hood ornaments
South Jeet Territory
North Jeet Territory
South Gutgash Territory
North Gutgash Territory
Pink Eye Territory
Join your hood ornaments and remember that you can place one in the front and one in the back of the car.
I hope i was clear if you have any question or suggestion regarding this achievement i will be glad to help you !