Has the blower motor been replaced? My experience is that the secondary market blowers do not work nearly as well as the OEM blower. I hope there is one out there that does work but I have not found it yet after three tries. For me, A/C and Defrost do not have the output needed to do the job. Using a relay is about the only alternative since max output with the newer motors is the equivalent of medium on the OEM blower.
Craig