I hope that this can help someone with a 1987-1991 Buick/Olds H body vehicle with manual ac/heater controls. The blower fan would only work on high, whereas it would bypass the heater resistors on low settings. All of the troubleshoot information I could find pointed to the #7 fuse in the dash fuse box or/and the resistor located in the blower enclosure. After replacing both, and checking the heater switch which had power at the connections with no fix, I was feeling a little defeated. Thank goodness for the internet. I found a forum where a guy had the same situation and was tired of throwing parts at it. He finally traced the problem to the blower relay, which would have been the cheapest part to buy except for the fuse. The resistor seems to be a common problem, but in my case as was the forum poster, it was the relay on the firewall, $ 1.50 from u-pull-it.