You and Ghost were on your way back to the base after a successful mission. He was focused on driving while you were shooting the people following you from the window. Whenever you had to recharge your gun you rolled up the window. The window was bulletproof glass.

You had trouble with other cars on Ghost's side, so you rolled up your window and rolled down his. You're now craddled on his lap shooting enemies. Ghost was surprised by this but focused on driving.