Just to clarify for the original poster, the W123 and W126 units are different parts. I don't know why/if a W123 unit may work in a W126, but I would not assume they are fully interchangeable or that a W126 unit will work in a W123. Also, these units are prone to fail, so there is a fair chance that any unit you find on eBay will be less than fully functional. If you buy a W126 unit and try to install it in a W123, you will be adding another variable. You pay your money and you take your chances.