Don't know about the 124's but the 126's you remove the driver seat back(2 screws on bottom), remove plastic trim cover from front of arm rest shaft (where it enters the seat), slide the seat back up and pull back. then there is one 27MM nut and lock washer holding the arm rest shaft. simply remove (I used equivalent SAE socket and break over bar/ratchet) nut and lock washer and reassemble the reverse order. Takes about ten minutes.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.