The OVP relay is a little silver box located up against the firewall near about 8-10 inches left of the fuse box. It is silver and has a red cap on top of it with a little 10 amp red fuse in the top with a clear plastic cover over it. Once you open the hood it is really easy to find if you know what it looks like.
1995 E420 - retired
1986 420SEL - retired