- Get a mule. You can either find one or get one off of the internet from Warstock
- Get into the mule and head over to the firestation
- once their you will see 3 garages, you will want to use the third one.
- Take the mule and back it straight into the second garage door, you're going to want to back it up.
- get on top of the mule and run straight towards the wall in front of you.
- Once at the end you will want to press X and then you will notice that you will climb up it.
Here is a video for a better visual: