Press 'B' to go to the brick selector, then click on the 'Special' tab.
Find the vehicle spawn, then click it and then click brick slot 1.
Exit out of the Brick menu by pressing 'B' again, then press the key '1' and click your mouse to where you want it to go, a ghost brick will appear. Press the number keys '8', '4', '6' and '2' to move the ghost brick around. When it is where you want it, press 'Enter'.
Then press 'Q', and scroll down to the Wrench. Hit the Vehicle Spawn with the wrench, then in the drop-down menu select 'Jeep' then press Send. A jeep will appear.
Use keys W, A, S, and D to move the Jeep, and move your mouse to reposition the camera.
Have fun ;-)