How to fix light dimming?

Just got an alpine mrp-m500 hooked to a type r. I have a stock 06 elantra 90A alternator. I just noticed while driving with a light or headlights on when a bass note hits the lights dim then return normal(haven't tested long notes). Also when idle the headlights dim which is how the car is made don't know if that has to do with anything but how can I fix this? Multiple options please