2002 Dodge Caravan heater fan only works on high.

BLOWER MOTOR RESISTOR
A blower motor resistor is used on this model when it is equipped with the manual HVAC control. See Fig. 5 or Fig. 6. Models equipped with the optional Automatic Temperature Control (ATC) use a blower power module, instead of the blower motor resistor. The blower motor resistor is installed in a mounting hole in the HVAC housing, directly behind the glove box opening of the instrument panel. The resistor consists of a molded plastic mounting plate with 2 integral connector receptacles. Concealed behind the mounting plate within the HVAC housing are 4 coiled resistor wires contained within a protective stamped steel cage. The resistor mounting plate is secured with 2 screws to the HVAC housing and is accessed for service by rolling down the glove box from the instrument panel. The blower motor resistor wires will get hot when in use. Do not touch the resistor wires or the protective cage if the blower motor has been running. The blower motor resistor cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.
The blower motor resistor is connected to the vehicle electrical system through a dedicated take out and connector of the instrument panel wire harness. A second connector receptacle receives the pigtail wire connector from the blower motor. The blower resistor gas multiple resistor wires, each of which will reduce the current flow through the blower motor to change the blower motor speed. The blower motor switch in the manual HVAC control directs the ground path for the blower motor through the correct resistor wire to obtain the selected speed. With the blower motor switch in the lowest speed position, the ground path for the motor is applied through all of the resistor wires. Each higher speed selected with the blower motor switch applies the blower motor ground path through fewer of the resistor wires, increasing the blower motor speed. When the blower motor switch is in the highest speed position, the blower motor resistor wires are bypassed and the blower motor receives a direct path to ground through the blower motor switch. The blower motor resistor may be diagnosed using conventional diagnostic tools and methods.