Ugh, my old Toyota had the weirdest electrical thing with its power locks.

Shared tools always reveal how people value their time versus others'. Your cousin sees cleanup as your problem, not his. This happens everywhere, from roommates leaving dishes to crews leaving a jobsite messy. Without clear rules, some will always push the work onto someone else. Try a posted checklist or a simple fine for not cleaning up. Letting it slide just teaches him that his time matters more than yours.