GEARSHIFTERS

During the first mission you should find some 'Schematics' - after the 3rd stage of the mission you'll be taken to the upgrade station where you can customise your vehicle. Select 'Secondary Weapon' then first equip the 'H-Mounting' - this will allow you to buy and install a secondary weapon on the roof. I had 'Rear Guns' and the 'Broadside Shotguns'. Equipping anything will unlock this.