Here is the full step by step instruction of my truck with trailer. More pictures are on http://www.instructables.com/id/Knex-Truck-with-Trailer/
At first the trailer step 1 till 3. And then the truck step 3 till 6.
Step 1: Trailer
At first the trailer axle for the wheels. Build it and put together. Per axle 2 blue spacers and 2 large wheels.