If you have a vehicle equipped with rear AC (SUVs and minivans often are), you need to take special consideration. These systems usually run a separate evaporator and expansion valve in the rear of the vehicle. This means there are lines that run all the way to the rear of the vehicle along with an extra set of HVAC controls to work them properly. If you're looking for the cause of a problem, be sure to check under the vehicle at the lines going to the rear, and also check the operation of the rear components. These systems often have a separate set of controls and might have their own blower motor as well. The best advice I can give is to not forget about these rear HVAC systems, especially if you have a van or SUV. You might also need to treat them as stand-alone systems when doing your diagnosis. If you have a service manual for your vehicle, this will be very helpful in your diagnosis.