Lights dim after speaker "system" was intstalled

Its my daughters car and the speaker system was installed by a reputalble company. She has also replaced the alternator and this did not solve the problem. Help!

that is perfectly normal, anyone who says different shouldn't try there hand at car audio at all... the problem is your stock electrical can't meet the demands of a stereo system an easy fix is doing the big 3 wiring upgrade just google it, or you can put another battery in parallel with the primary which is what most bass heads will do(i am one) hope this helps. any questions and i will gladly help you my phone number is 1-256-324-0771 and my name is adam