do you have any kind of like aftermarket stereo equipment in the vehicle? did you double check the wires going from the alt to the wiring harness??? Possibly bad perhaps? Start at the alt and work your way up. It sucks you spent 600 on it already and haven't fixed the problem. Next time tell them that they haven't done their job and you won't pay them untill they fix it. 600 is more than enought to fix a problem.