It could be the alternator. Take it back have it tested and demand another. They probably overtightened modified or broke part of the wiring harness to the alternator. If it runs okay it sounds like the ground is effed up and its not making proper contact. It's a wiring/grounding issue with the engine/alternator and car body. Most mechanics can't do anything more than hang parts when it comes to electric problems. Whoever said anything about a module is clueless or trying to rip you off. Don't let them sell you a computer. Go to another mechanic if they say that. Look at the wires plugged into the alternator for splices, then check the alternator contacts for corrosion. If you have an ohm meter see if there is resistance between the negative battery terminal and the alternator body or engine. Finally, test the battery voltage. It should be around 13.5 volts with the engine running. If its not, you have a bad alternator or a short.
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