Have them do a complete charging system test. They do , do that don't they? Probably alternator. Also check for a parasitic draw. Shouldn't be more then 50mA. If it does start pulling one fuse at a time until you reach a desirable limit. Anything above 50mA will drain your battery.

When you go to test the charging system make sure you battery (if its dead now recharge it) fullly or acceptable.