I had a 150w Kenwood amp running a 10" kenwood sub, whenever the bass hit, the headlights would dim, and this was in an isuzu rodeo. I put in a 1/2 F cap and it went away. I'm not sure if it was my altenator or not, as I've recently had to replace it, and haven't tried it without the cap in.

Edit: even now, when I roll the windows all the way up and keep the buttons pressed, all of the lights dim.. but only once the windows are up..