Your landing paper

If you take air flights, greyhound etc commercial transporations, you need to show your maple leaf card. In your case, you have to drive up by yourself, show them your landing paper. Besides, did you report import goods when you landed in Canada last Dec.? Did you put your vehicle on the import goods list? If you did, you should be ok although they'll ask you a bunch of questions. Otherwise, you have to explain why you are driving vehicle with US plate.