Replacing the left I/O board seems to be the reasonable course of action at this point. Here's how to do it: [guide|504]
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Here's the part to do it: http://www.ifixit.com/MacBook-Parts/MacBook-Pro-15-Inch-Model-A1150-Left-I-O-Board/IF185-016?utm_source=ifixit_guide&utm_medium=guide_intro&utm_content=required_items&utm_term=macbook_pro_15%22_core_duo_model_a1150