I will give you the series chain so you can diagnose after you take the readings.

The power goes from Bat + > Ign sw > F15/30 A. fuse link > to Red of firewall connector > blower +.
The yl comes from CP > regulator. That is reg trigger signal and is fused via #7.

If you have 12 v at red/blk and trigger at yl/blk at firewall, then the problem is down at the blower/reg.
Once there , you jumper a wire from the blu blower motor wire to ground. If that gives High Fan, the Regulator is bad.
Notice that the switching for the blower is done on the NEG side [ It is a Switched Ground Circuit], where the RED to blower is Allways HOT w/KO...
Grounding he blu wire from blower By=passes the Regulator, and therefor, that is the Regulator test. If no Variable V signal at Yl , then the reg will not trigger and No FAN can happen.

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A Dalton