The biggest problem here is that many components are talked about as if they all existed on all 124 cars. As it turns out the OVP function was taken over by the MAS (not mass air flow). MAS was a combination OVP, KLIMA and FPR (fuel pump relay). The v8s changed everything. A whole new electrical center is setting where the battery used to and all the controllers including the base module which is an even more sophisticated OVR/Klima/FPR/air pump relay/etc. See illustration.
The relay under the seat is the fuel pump relay, I think. Which is the same relay as the aux fan relay one stage or the other, which is located in the fuse panel relay center.
Bosch Master, ASE Master, L1
33 years MB technician