I've found you need to get most of the trailer within the box and your truck cab behind the white line, then it should turn green. #1. Scania84 Jul 21, 2013 @ 4:13am. Press T when the box turns green or if you don't want to park the trailer just press enter. Hope this helps. Last edited by Scania84 ; Jul 21, 2013 @ 4:14am. 6PRvZ4Y.