Instructions
Go to Manzanita Post. It is located in the western part of West Elizabeth in the northeast corner of the world map.
Find the General Store in Manzanita Post. It is called "Manzanita Trading Post" and is located to the left of the town entrance. As you near the storefront, there is a chance you will randomly encounter a woman who will ask you to help her find a lost supply wagon from her convoy. This is a random encounter -- there is no way to guarantee it will take place. If it does not happen as you approach the storefront, leave Manzanita Post and come back later. Repeat until you are approached.
Agree to find the missing supply wagon when prompted to do so. The woman will give you the location the wagon was last seen, either north or south of Manzanita Post.
Leave Manzanita Post by the main road and head north or south at the first fork, depending on the direction the woman specified. Stay on the road; leaving it will cancel the mission, and you will not be able to find the missing supply wagon.
Look for vultures circling in the sky in the direction the woman sent you as you travel. You will see them within a few minutes after leaving Manzanita Post. Go to the location above which they are circling. The missing supply wagon is there, a few dozen yards to the side of the main road. Be prepared to fight when you arrive.
Kill the wagon robber guarding the missing supply wagon. He might be hiding behind the wagon as you approach. When he is dead, take control of the supply wagon and drive it to Manzanita Post for a reward.