hi
i have done this mod i used 15 copper tube spaced the right distance apart with a metal plate on the top i soldered 2 olives to the pipe then placed through the metal plate (3mm) then with the 4 mm above the metal plate hammered it over like a rivot
i marked the seat back for position and made 2 cross slits to take the head rest through the seat back material i did this before unpicking the seat material back to place the plate with tubing in the seat back under the seat material its also good to use a piece of tube heated up to burn through the foam to make space for the plate/tube once in position put it all back in and slid the head rest in but if i remember i made wedges to go in tube to make stiff so head rest was sturdy
once i had had the seats apart i see another way to do the mod from the back of the seat without unpicking the seat material and have done it a few times and is alot better
it entails making 2 brackets for each headrest and a hole at the back of the seat with locking bolts to clamp headrest in postion and does not notice but this is to hard to explain how todo as you have to be a good welder to make the brackets and and have the markings right on the back seat for it to line up right if you follow me
mark2