This is not really a cheat but works 110%, trust me. You don't have to have unlocked the 3rd island to do this, all you have to do is travel to an area on your map that says 'The Big Ear' (the big satellite near Area 51).

Nearby should be a car park with just a motorbike, a Freeway I think, but it has to be that one.

You get on the freeway then drive it into area 51, when you get to area 51 there should be a solar panel thing on a slope.

Position the freeway right next to it so when you get off your getting off the side the solar panel thing is at, you should then fall through the solar panel thing into Area 51.

There isn't really anything to do inside Area 51 until you do the airstrip missions involving truth, which needs you to go inside.

I haven't done the mission myself but I know of it.

NOTE: It has to be the freeway in the carpark next to the big ear or it wont work!