I have a 1998 GMC Sonoma that has no heat and the blower mower hums. I have changed the relay switch and the resister. Also the third brake light comes on the the heat setting is on 3.
Sounds like the blower motor is bad. Since it hums we know it's getting power. I'd replace the blower motor. As for the 3rd brake light, that's a new one on me. I'd have to say someone wired it in to get it to work. Just so happens they picked the wire for speed 3 on the blower switch. HTH. -Jim
The blower motor is good, the relay and resister are both new. The humming comes out of the relay on speed 1,2, and 3 the only thing that occurs on speed 4 is the 3rd brake light comes on.
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