The rear AC blows fine and is cold, but the front AC flow is low and is not all that cold. Both units obviously run off the same compressor and other under-the-hood components, but do they use the same evaporator, etc.? Given that I live on dirt roads where dust is high, is there a filter that I should clean or is the evaporator clogged? Perhaps it is even a "flapper" issue between inside/outside air. I'm lost, where should I begin?