Jiggling the key doesn't help....there is just no high fan speed, whether in manual or automatic mode.

Unlike the W140, a replacement blower motor for the R129 does not come equipped with a regulator. But is it prudent to replace the motor when a new regulator is installed?

I assume blower motor function could easily be determined by supplying 12 v direct and checking function and amperage draw; high speed operation in the absence of the regulator would rule out a blower motor problem, provided the amperage is within specs.

Could a pushbutton control unit malfunction also result in loss of high fan speed, while leaving the three lower speeds functional?