How about the simple explanations for high beam bulbs which don't come on, in any mode (DRL or not):
I'm not saying that it is one of these, but it makes sense to me to check the most obvious causes - and easiest to check - first.
- Are the bulbs burned out?
- they are completely separate from the low beams, and the low beams stay on when the high beams are turned on, so someone driving only in city could go for a long time and not notice
- Are the high beam fuses blown?
- there is a separate fuse for each, used for nothing else so someone not using high beams would not notice
I'm not saying that it is one of these, but it makes sense to me to check the most obvious causes - and easiest to check - first.