Hi Alexander, unfortunately a few things could be a play. I haven't experienced this particular issue on the XTS, but had a 2008 DTS that had a similar headlight problems. Long story short, I found each headlamp had a high voltage ballast that misbehaved when either the alternator's voltage crept up a little high (say, past +15VDC) or if the battery had a surface charge on it. Additionally, pending on the bulbs you used, they may be drawing excessive current, enough to cause your ballasts to shut off altogether. I had a problem with a ballast (again, on the DTS) that had a leaky ground, causing the ballast to shut off altogether.