For this achievement you have to find the blueprint and build 10 combo cars. The below list tells you the vehicles needed for the standard combo, but you can use attribute points to unlock other variations . I had 10/11 vehicle blueprints found when I got the achievement.
They are:
1) Shockdozer (Backhoe Loader & Ambulance)
2) Junk Bike (Motorcycle & Sedan)
3) Party Slapper (Street Sweeper & Party Van)
4) Junk Truck (Van & Suv)
5) Rollerhawg (Motorcycle & Steamroller)
6) Forkwork (Forklift & Fireworks Van)
7) Minibike (Motorcycle & Sports Bike)
8) Turret Rig (Steamroller & Sedan)
9) Junk Car (Sedan & Muscle Car)
10) Mobile Locker*(Van & Van)
*The Mobile Locker isn't available to be built until you hit level 50 and upgrade your inventory all the way.
If you've missed some of these blueprints, Alcoholic Socks tells you where they are located on the Master Builder Achievement.