hi. these are some photos of my rear headrests. it was easy. this is for a 90, I don't know how the 110 seats are made.
basically the backrest is a square. I cut out a couple of inches so that I can insert a vertical channel for headrest to slide up and down in. welded everything in and powder coated.
backrest frame is very flimsy and this made it a little stiffer.
the channel I used was just scrap from a range rover seat track. you are gone have to bring in your headrest to a metal supplier and find a channel that fits it.
the first photo is bottom of frame and 2 others are at top.