By the way, the solenoid on your starter is another word for "big damn relay"! Hate to have that much current flowing around my hand when I engage the ignition...
Anyway, the OVP is located behind the battery. There is a black plastic shield behind the battery (held in place by form only), that can be removed simply by pressing near the top of the lip and moving it out of the way of the firewall.
From what I've read in earlier posts, you would now be staring at three units, one that controls the fuel pump, another for the air conditioning, and the OVP. In early models, it ha a red top with one or two fuses (mine has a green top). I am waiting for my OVP to come in the mail shortly, so I can't tell you anything else about replacement until it comes in.
I am sure more replies from other more-experienced owners will follow.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car