Hello XXXXXXX, my name is Mike.
I have not had this problem nor is there any bullitens on the issue. The light assemblies have tabs in the bottom that slip into holes in retainers then they are bolted in from the top. If the tabs are not properly in their holes vibration will occour. The only other thing I can think of to look at is the internals of the lamp assembly. The bulb is replaceable and if it is not fully installed in the assembly it will bounce. The light deflector placed around the bulb could also break free and vibrate.
Other than that if there is any looseness in the points that the assembly bolts to or sits in vibration will occour.
ASE Certified Technician
Seventeen years of automotive experience with the past thirteen in General Motors dealerships.
Thank you for accepting XXXXXXX, I wanted to add something to my previous post.
If you are not satisfied by your current dealer you can take it to another dealer. I don't intend to steer you away from your current one that is just to let you know that is an option.
Let me give some advice that might help the technician see your problem. If you are in a daylight savings zone it should be dark by closing time. Ask the service advisor to go for a drive with you when it is dark and demonstrate your concern. If someone sees what your problem is you have a much better chance of having someone look into it until the problem is resolved.