1992 GMC Sonoma Speed control for fan blower

Hello sirs, my speed contol from low to high blower speed will not funtion correctly for heater or air condition inside cab. Have replaced the relay switch underneath the hood on firewall. Did not correct. I can leave air conditioner on max cool and fan on high setting with car running or not, have to tap the relay under hood. Blower will start blowing on high or where switch button is set. Will keep blowing till I set to lower setting. If I change setting back to high etc, no responce. Air conditioner is colling just fine, just can not control the air blowing speed inside. Thanks for any suggestions.