You have to monitor the voltage at the yellow wire at the 3 prong plug on firewall, behind Brake booster when condition exist. That is the variable trigger voltage input to blower regulator..if that voltage changes and blower speed responds , the problem is upstream from there [ Control section of circuit]..if blower changes speed and that voltage remains constant, the problem is down streams from there ..[ reg/blower section of circuit]
You also can check that same 3 wire plug for constant 12v across blk/red wires ..if those drop below system voltage, you could have either a bad Ign. Sw/ poor ground/ bad connection at fuse ..[ they are infamous for hairline cracks/poor connection]