I think the rear headrests were deliberately locked upright in the 7 seat versions as they also serve as headrests (or more accurately, head restraints) for the third row seats.
A strange Australian design rule around the time of our cars meant that the front headrests in both our 300TE and the 190E were also locked in the upright position. Models delivered here from about '93 onwards allowed the headrests to tilt forward as in other markets. Fortunately, with a bit of poking around in the bottom of the headrests it is possible to remove the metal clip that was added to prevent them tilting.
I had considered checking to see if the rear headrests in the wagon used the same clip although decided for safety reasons to leave them locked upright since we do occasionally use the third row seats.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.