"Tapping" into the existing speaker system...

Can anyone tell me if there is a way to tap into the existing speaker system in my 2000 Pontiac Montana to use with my portable DVD Player? I would like to use it in the same way Montanavision uses it. My vehicle did not come with that option but I have a Portable DVD Player that I hope I can adapt. Is there a connector that I can buy somewhere? Even if someone can tell me which wires to splice, I can even do it that way. Thanks.