I've never been a fan of high-beam DRLs, even at reduced intensity. They're still the high beam, and they're still right into oncoming eyes. That's safer exactly how? It makes a really big difference at dusk or heavy cloud cover, when you'd think a bright person would just turn on their lights, but they don't. The system stays on DRLs, and now they're just too bright against a darkish environment. Notice that early GM DRLs were 85% high-beams, but that has long since been abandoned in favor of dedicated lamps.
If you have to try this, use the low beams and forget about intensity reduction. The idea of DRLs, after all, is to be seen, and the fact that most drivers forget to turn on lights in the rain, or too late at dusk, means that always having low beams on would be a good thing. Gee, just like a bike!
I got stuck behind a tank truck in northern New Mexico for nearly an hour, because I couldn't see unlit cars through the spray. I saw every car with lights on well ahead, but a dark car was always a surprise, so I followed this guy at 45 or 50 for 30-odd miles. On a flat and straight road. DRLs on every car would have made that an easy pass.
As for the electrics, a relay on the ignition circuit could turn your lights on, but won't force them to low beam (except some late-80's/early 90's Mazda and Fords I've seen - lights ALWAYS turned on at low beam). So you want the relay to go right to the low beam lamps, not through the headlight switch, and you want the headlight switch to disable the DRL circuit when you turn the lights on manually. How exactly depends on the existing light wiring, where the relays are, etc.
