Both sentences are correct and have exactly the same meaning in German. Albeit, the first one

Der Wagen ist mein.

is very rarely used and might be interpreted as wrong (see the comments).

Incorrect, but often used and probably correct in some future:

Der Wagen ist mir.

Both sentences are correct and have exactly the same meaning in German. Albeit, the first one

Der Wagen ist mein.

is very rarely used and might be interpreted as wrong (see the comments).

In some Upper and Central German dialects, one can also hear

Der Wagen ist mir.

but this is not part of Standard German.