I'm using a climate control from a 126 in my '84 300D. I can tell you the electronics are not identical; the fan sounds a little different and doesn't start and stop exactly the same, but it seems to work fine, for heat. I don't have an AC compressor, and don't use it for that purpose, so I'm only endorsing 50% functionality, really. But I suspect they will interchange. The part numbers are only different by like one digit too. One forum member told me the only difference is that the 123 version comes with the brass clips on the top for mounting, and the 126 version didn't. That's how it worked out for me, but that's just anectdotal.

For what it's worth,
Ben