My worst mod was the very first one I ever did. Polaris radio shade cover. Still can't believe I spent almost $30 for this 10¢ piece of plastic. As worthless as tits on a bull.
I just received my '16 SL about a week ago and haven't done much yet but I can attest: The worst mod the previous owners did was the Assault e-brake handle cover. Terrible in the hand and difficult to engage. We live in an extremely hilly area and I generally use the e-brake to launch uphill. Very difficult to do with this erector set lookalike on your e-brake handle. Removed immediately and am waiting on another.