First, you need to make sure the correct bulbs were used.....
Second, need to know that they were installed correctly(no fingers touching the bulb!)....
Third, look for water in the lens capsule.....
As for the alternator, sometimes a code will be set.....it may not turn on the CE light.....Does the dash have a voltage gage? If it does, compare it to a voltmeter across the battery.....should be the same....watch gage for jumps while driving, especially when turning "loads" on the system on/off....
Alternators can be tricky......they may surge in a split second, and you wouldn't even notice it....the sad part is, they may may not be "surging" while you test them.....I would check the first three items again......then, check the connections at the alternator that they are clean and tight.....
Just for ha-ha's, take the vehicle to a place like Autozone, where they can check for codes in the car for free.....if you find a high voltage code, it could be the alternator.....but it could also be the PCM(computer), or the BCM(body control module), whichever one regulates the alternator....
Another possibility, is to go to a local GM garage, and ask them to look up TSB's, to see if there is a bulletin on your year G6, for this problem occurring....