1997 Blazer fan doesn't work on high
 

I replaced the blower motor resistor. (I figured I'd get that out of the way first.) I replaced that part after having searched here and that seemed to be the wisdom of the group. Now I'm seeing that the blower motor resistor controls settings 1,2,&3, but not 4/high.

I looked haphazardly at the fuses and didn't see anything. I've replaced the b.m.r. I figure it could be the switch, a short, or the motor. Am I right?

How do I go about troubleshooting this the least inconvenient way? I've got a multimeter but I'll probably have to dust it off, ...so go easy.

The blower motor is obviously OK because it works on 1, 2, & 3. The high speed blower relay is located on the firewall, toward the passenger side of center. It's about a 1" cube, black plastic with 5 wires attached to it. All 4 blower speeds go through the relay, but when you select HIGH, the relay latches and supplies the blower motor with battery voltage.

Dark blue: Supplies voltage to the blower motor from the resistor for speeds 1, 2, & 3. It's obviously OK because you have all of the low speeds.
Purple: Goes from the relay to the blower motor, it's also OK because the blower motor works.
Black: Splices to the blower motor ground wire and grounds to the rear of the right cylinder head. This is also good on yours.

Here's where you'll find the problem:
Orange: With the ignition ON, the mode switch set on HEATER, the blower switch on HIGH speed, it must show battery voltage.
Red: Must show battery voltage at all times.

If the orange and red wires check out OK, the relay is faulty. If either or both fail, post your results.

you replace the heater control?(the control under the radio) fairly common thing to go if you have all speeds but one