an only works on high cannot control speed

The relay is as good a starting point as any. I'm attaching screenshots of the fuse box information from AllData. Look at relay R06. If that relay matches any others in there, then you can try subbing in another one to see if anything changes. If not, then we'll need to look at the Blower Resistor, which has a hand of it's own in controlling blower speed. Let us know what you find please. Good luck!

I agree with Joe. The resistor is the most likely cause of the issue. It's an easy fix, and not a terribly expensive part. Let us know how it works out. Good luck!