It's hard to say for sure but it looks like the MB543648 just has a metal mounting tab that is not on the MB543647. I compared the pin out and diagram between the head light popup motor relay and the door lock power relay and they are the same functionally.
It looks like the MB543648 is used on cars that have a remote for the door locks and is located below the receiver under the trim panel on the drivers side B column. The MB543647 are located either under the hood for the popup or on the drivers side footwell near the driver side fuse box for the door lock on cars without the remote.
