Yes there should be power at the pump at all times. The rectangular connector is the pump power source, the round 3 pin connector with indent are the feeds from the doors and trunk.

As stated above sounds like a bad connection or broken wire. If it cant be found, just re route a new feed line. Some of these cars have a fair amount of wires in them.

One of my cars, I found a broken wire right where the loom comes into the trunk beside the spare tire, it was hidden underneath the wire cover.

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Just my MB's:
1990 420sel - Winter DD - 410,000km
1990 560sel - absolutely showroom mint - 360,000km!