Only high speed with blower...help!

I'd check the circuit from the blower resistor connector with a multimeter. With the key in the ON position and the connector unplugged and in your hand, you should have 12v (battery voltage) on the Blue/White wire. Then I check for resistance to ground on the remaining 3 wires. With the fan switch in postion 1(Low), the Blue/Black wire should have 0 ohms of resistance to ground. With the fan switch in postion 2, the Blue/Red wire should have 0 ohms of resistance to ground. With the fan switch in postion 3, the Blue/Yellow wire should have 0 ohms of resistance to ground. If all of these checks are good, your new blower motor resistor is defective.