Can you change the blower speed at all?

The blower motor resistor may be the problem. There are several posts on this issue that might help you troubleshoot. It has an updated part, so if you need to replace you'll have to replace the blower motor with an updated part too ($$). It is accessible from underneath dash on the passenger side. The only hard part is lying on your back in the passenger-side foot well.

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1998 E320 120K mi
1991 Miata 80K mi
2001 Ranger 109K mi