Frau

The 400E fuel pump relay is located under the passenger seat, its on the right side of the seat as I recall.

The 400E does not have an OVP relay - the function of the OVP relay is, I believe, performed by the base module. There is a fuse in the module that will blow but I don't think the idle is effected by that fuse as OVP failure is with the K-jet systems.

I would check fuel pressure and read out the codes in the ECM as a starting point. You may also want to check the caps,rotors and plug wires too. I'm not sure if relay failures will set a diagnostic code - maybe someone with the M119 codes could check.

My 400e had an intermittent rough idle/stalling problem that disappeared when I changed the infamous engine wiring harness. I was hopefull that might be your problem but I see you have already changed it.