The PO stuck another mat on top of the mat that's already in there. I can't think of any other reason to stack one mat on top of another other than to put gentle pressure on the mat to keep it still. Thus, I haven't had any issues of the mat sliding around.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.