Mystery Rx7

That would depend on where you are in the single player/storyline. You have to progress to unlock the different cars OR you can steal one of the abandoned cars when the RX-7 comes up in the rotation again. Hope this helps.

You have to find it and take it to the airfield without getting busted, this week it's located right in front of the player's garage on the west side of the map.

It'd a drag build so it should be pretty easy.