I don't know if anyone's done this on a W124. I can't imagine it would be any different. the only real difference between the W124 and W126 is that the W124 pump is under the rear seat.
I prefer the Crutchfield solution myself since I can't solder to save my life and the benefit of Crutchfield support seems to be a worth the extra money.
81 300SD (on ebay)