Check the contacts where the OVP and fuel pump relay plug in and make sure they are clean and contact properly.
There is an earthing point on the inlet manifold adjacent the IACV. (A bunch of Brown wires screwed to the manifold). Take the bolt out the fastens the brown wires to the manifold. clean off an corrosion, clean the tags on the end of the wiring, clean the corrosion out of the hole where the bolt goes in.

With the engine running you should see 13.6 - 13.8 volts across the battery. This is the output of the alternator regulator. This was the charging volatge for the simple lead acid batteries which were prevalent during the era of the design of the W124.

Underneath the car there is a flat wire copper rope strap between the body and the rear of the engine. This is the earthing point. Make sure there is no corrosion anywhere here.