Turns out all that needed replacement is the OVP FUSE -- the OVP relay itself seems fine.
On a 300CE (W124), the OVP is located (along with other electrical controls) in a compartment behind the battery. Its covered by a black plastic shroud -- lift this off to reveal the OVP relay.
Its a beige relay, with a clear flip-up fuse cover.
Picture of the OVP relay below