Diesel Benz
MB Enthusiast
In theory reverse polarity on a relay would not cause it to fail as it is just a coil so polarity is not really important.
Maybe was just gubbed anyways
The relay would not be driven directly from battery terminals (when reverse connected). How would the relay get reverse control, the computers would not run backwards and feed negative drive when battery is reversed.
Some relays have a diode suppressing the switch-off surge. Better to avoid reverse voltage if not sure about the part.
Better to wait for the OP explaining what he did as the problem was fixed.