When your removed the seat from the M5 1995, you were forced to leave in that car the Memory control module connector X18322 (Black, 26 pins), and connector X18323 (Black, 4 pins). One of them had attached wires going under the carpet, to connectors in left and right kick side panels, just ahead of the front doors, and the other, had a brown/orange wire bolted to ground under the rear seat. As you did, you also leaved the male half part of connector X275 (Black, 12 pins) wired to those fore mentioned connectors, this was the mating part for the connector under the carpet, at driver’s seat of your 1989 E34. You probably have hanging from the seat the male half of connector X626 (Black 8 pin), the female half went away with the X275 male half. This all suppose you did not cut any wires.
If you look at your new seat, you should have a black box (Memory seat mirror control module) showing two missing plugs (X18322, and X18323, total 30 pins). In these conditions, to keep the memory feature for the seat requires to connect to those pins without the proper connecting plug, which, though possible, is some how complicated. The handicap I see here is that you are missing all the Mirror control devices. The mirrors control switch is a special one, and also the mirrors themselves are special. My concern is how will the system behave if he found no feedback signal from the mirror position indicating potentiometers. I do not want to bias your decision, but would be much easier to remove the memory feature from the seat and convert it to a plain power operated seat. I wouldn’t do it, but rather go for the mirrors, the mirror switch control and all the missing harnesses to complete the system.
Let me know what you want to do. If you decide to bypass the memory, I can prepare you a detailed chart for wire cut and splice. The only limitation will be the connection to your existing connector under the seat; I wouldn’t like to cut the wires going to it, to splice them into the seat wires, as you will not be able to unplug them when attempting to remove the seat for maintenance/cleaning purposes. Think the best chance would be to obtain a male/female up to eight (depends on the seat features) wires connector and replace the existing X275 under the seat. Also let me know if the seat has thigh support switch, lumbar support switch, and heater switch.
Javier
PS: Haven’t seen any pictures yet from under your new seat.