Electric rears were always a monogram option (on both MkI and MkII) and I think the facelift 160 had electric rears as standard. I doubt if the wiring is there if electric windows are not already fitted - MGR couldn't afford to fit anything that was not needed.
It would be possible to get them fitted, but would need the wiring and switches, relays and of course the door motors and associated gubbins. These would be very expensive from XPart (if still available), and hard to find in a scrappies due to their rarity. If you can find the parts, it would be a good mod.