I added the rear headrests using OEM parts, myself. They are doglegged I believe.

Rear headrests were a very rare option on the 109s. They are easy to mount -- you just have to pull the back seat out and mount the brackets inside the seat, punch holes through the top of the leather in the seatback, and mount them. I just (3 years after the fact) got the little black plastic rings that go in the holes in the leather, to finish the effect. Same as a 116.

Cheers,
Gerry