PM matt16v and he'll tell you, we did it on his but i didn't take much notice.
We anchored the side ones (door side) to the bit that hold the bottom of the seatbelt there, and i think the other one to where the seatbelt plug sat. Shows on picture. Bottom left and bottom right of the seats.
but one wouldn't work so we ran in under the carpet and did it to the other side.
At the e38 i saw some people attach the rear one to the very back seatbelt golder. (above parcel shelf) Looked ok.
