If you have an original set of back-up lights, the grommets can be cut off and transferred to the new ones. Actually, if the whole original harness is in good condition, they can be removed at the socket and completely transferred! I did this on the K convertible.
Going back to splitting the grommet... you will see a mold line somewhere on the grommet. Use a razor blade to split along the line. Then use a little glue and stick the piece back together. The pressure of the hole will keep the grommet from coming apart also.