Open both plug on the van and socket on the car and check for discolourisation on the wires. The insulation can shrink back in extreme cases and leave exposed conductors when they get hot from high resistance joins. If this is the case trims the wires back to good insulation and reterminate. If colours look good check the terminal screws are tight. A high resistance joint can be a loose terminal screw, corrosion on the pins and socket or loose pin in the socket and the latter can be fixed by levering with a screw driver the pins where they have a manufactured split.