Try to cut a deal with him. Tell him that one month, you’ll clean out the car, but the next month it’s his turn. Offer to go first. Or, make some quality time of it. Ask if he’d like to clean the car out together, then you’ll take him out for a burger or something. If he doesn’t want to, then start playing dirty. Leave your trash in there until he can’t stand it anymore. And leave him with absolutely no gas. Those together could start infringing on his relationship with his girlfriend, which is usually a breaking point.