can the front seats on the 90 420 sel be moved fore and aft manually? if so, how

If you need to get at the bolts at a boneyard, a 12 volt battery from a Makita screw gun works great. You just have to figure out which wires you need.

That's not exactly manually but not sure there's any other way. A big downside of these new fangled contraptions.

I made a seat switch out of 4 BMW window switches. Took some doing with a continuity tester to dissect the working of the switches but it works great. Thread about it here.

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1986 300SDL, 362K
1984 300D, 138K