W126's use an electrical pickup at the output of the transmission. W123's use a cable so look around where the driveshaft meets the transmission. I've never been under a 126 but on my 123, that's where the speedo cable comes out.

__________________
Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo