Mission: OligarchThe garage can be found on the North East side of the island in a separate building just outside the main mansion. If you don't have any explosives, don't fret, you'll find an orange weapon case inside the garage, housing a Gold AK & Grenade Launcher attachment.
As you start the mission swim North to the main Jetty just below the mansion, head up the staircase towards the mansion itself. You'll reach a locked gate, take a right and mantle over the rocks to get around the locked gate. Continue up the stairs, disabling the 3x C4 booby traps as you make your way up. As you reach the top go round the right side of the mansion and you'll see the garage directly in front of you. In the middle of the garage you'll see an orange weapon case, loot the Gold AK and use it's grenade launcher attachment to destroy all 6 cars.