My daughter has a 1990 Blazer. Here's the issue. The heater/fan only works/blows when on the accelerator. The fan does not blow when it is idling. Additionally, the fan does not blow at all on low or high, only in the middle of the two speeds. Any ideas?

Thanks in advance!

Welcome to AF!

Check your connections at the blower motor, the switch inside, and the connection at the resisto pack, under the hood. See what you get. This sounds like you have a loose connection.