In my owner's manual, there is a section on how to lift the car. Have you checked you owner's manual? I think it may also be on the service CD.

For a 420 SEL, if I recall correctly, for the front it recommends the engine cross brace, and for the rear, the differential, in addition to the four rubber pad jack points on the frame. I heard some member may have damaged their car by lifting on the rear differential, so if you try searching on "differential lifting", you may find some details. Also, depending on what you are doing, maybe drive-up ramps will be ok. But please don't trust your life to my memory.

__________________
1989 420 SEL