Though your car is a W126 vs. my W116, there should be some similarities. For the front I would place the jackstands where that cross brace is welded to the car. If it's like mine, it's basicly a large tube that stretches across the car. For the back, I would use the point where the trailing A arm bolts up. Not the trailing A arm itself but, where it bolts to the floor pan. What ever you do, DO NOT places jackstands on the rear axle half shafts. You can't really place a jackstand on the rear differential itself without the car tipping one direction or the other. This would not be a good thing.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member