You do need to change the module and harness. The w124 and w126 use the same modules. Read through this thread.

W124 Passenger Airbag Retrofit Project Completed (with pics)

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http://i284.photobucket.com/albums/l...aman/Fleet.jpg

Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic