What kind of headlights to you have - the small round lens type with the Xenon HID bulbs within or the larger type with the internal reflector and halogen bulbs?
The problem I had was rear tail lights or the brake light bulbs burning out more often than I thought they should - perhaps every six months or so - not all of them all at the same time - just continually it seemed replacing rear bulbs. My HIDs were OK, but if you have the halogen bulbs, there are not much different than a conventional tungsten bulb - just the gas.
At some point I decided my alternator was not working properly, (not due to the bulb problem), but more because the alternator voltage never varied. ( I have a ScanGauge voltage readout device mounted on the dash.)
Bottom line was I eventually replaced the alternator and now it is rare I have to change tail/brake light bulbs.
I believe the regulator within the alternator was defective. There is apparently a fail safe voltage ~13.3 VDC that the regulator defaults to that allows the alternator to continue to generate amps sufficient to charge and generally run the 3 for months on end. While all seems well, it is not true.
In other words, you may have a defective regulator located within the alternator that is masking a problem.
Within the link below, there are a number of files related to the alternator, regulator, and the electrical system generally.
https://www.disco3.co.uk/gallery/thumbnails.php?album=3801