I use Glass Cleaner with ammonia. Saturate the carpet, get out a stiff brush and work over the trouble spots. Fire up the air compressor and use 120psi air to blow all the dust and dirt out of the carpet. I have brought carpets back from the dead using this method. I got a 3 year old coffee stain out of my FIL's Nissan. It was baaad too and everyone told me there was not any way to get it out.