Whenever you get inside of a vehicle, you can press the number buttons (1,2,3, etc) to change which seat you are in while in the vehicle. For instance, if you are playing instant action and a bot beats you to the hellbender and starts to drive away, hop into the side or rear turret and once you are in the vehicle press 1. The bot will switch seats with you and let you drive...To switch between seats though just use the number buttons
Technically it's not # keys but WEAPON keys. Shield gun (usually 1) puts you in driver, Assault Rifle, Biorifle, Shock Rifle and Link Gun keys put you in seats 2-5 respectively.
I found that out when I rebound my guns to have the RL / Flak / Shock as 1/2/3.
Comment