1995 Monte Carlo Blower Motor

My blower fan has been doing some strange things. The first 3 settings of the fan function fine, but the highest setting kills the fan. I noticed that when I turn the switch to the highest setting, the blower relay under the dash clicked. However, it didn't click on any of the other settings. This led me to replace the relay, but the new relay also clicks on the 4th setting. Today I discovered that the voltage to the blower fan is the same at all settings on the switch, even the fourth. It is actually the current that changes as you turn up the switch one turn at a time. The fan doesn't turn on at the fourth setting because there is no current to the fan at the fourth setting.

Anyone know what I should replace next?