Instructions
Start "The Storm" mission from the campaign menu. Start at the very beginning of the mission, not at any of the checkpoints.
Enter the driver seat of one of the Warthogs located at the starting point. (If you were to enter the passenger or gunner seat, an AI teammate would start driving for you, and would quickly move past the area you need to reach.)
Drive to the small ramp located just to your left as you exit the starting area. It has yellow lines marking the top and bottom of the ramp.
Park the Warthog so the front wheels are just a little past the yellow line at the top of the ramp. When correctly positioned, the light reflecting off the Warthog at this point causes it to appear gold.
Inch the Warthog back and forth over the yellow line if it does not change color. The positioning for this if fairly precise, so it may take a few attempts to get properly positioned.