CKO In-Car DVD Player Distortion (CKOMMP001)

Question

Hello Everyone This is my problem I have a CKO In-Car DVD Player 1/2din which get distorted when I turn it up?I can?t turn the volume no more the (1 Bar) The car that I am using is a BMW Mini Cooper S I have the BMW upgrade Harman Kardon audio system, which has a powerful amp..Because the DVD player has a amp and the head unit also has a amp?the is being amplified to many times in distorts.Can anyone suggestion something that I could use between the DVD player & the head unit to reduce the distortion maybe something that reduces the power of the DVD player out put so I could turn it up and get a better all round sound.Many Thanks David