This is a mission given to you by Hope, either from Gutgash's or Deep Friah's Stronghold (depending on your progression). She tells you about a car owned by the bikers, hidden inside a secret location. She tells you to go over there and take it for yourself.
After reaching the marked location, you have to enter into a tunnel. Soon, you will reach a huge cave within it. A pickup truck with a motorbike behind it will be slightly suspended with some chains. You have to refuel the truck and then, drive it out of the tunnel by boosting into 3 weak wooden roadblocks. Then, bring it to the closest stronghold (preferably Gutgash's) and you will get a new car to ride.
The "Demented Chariot" becomes part of your vehicle collection. A sweet pickup truck which has a skeleton riding a bike (a chopper) it's back.