Looks to me a bit like an electrical loom, not a drive cable - maybe a bad picture. If the seat will still go up you may get enough clearance for a spanner by using that moror. In any case you will have more space under the seat to wind the place where the broken cable is. You will need to make 50 or 60 turns to move the seat and you have to keep the seat rails 'together' or the seat twist will stop you moving the disconnected part of the mechanism..
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i