Since my mom bought the most basic model we only have 8 speakers. My mom wants to add speakers in the very back because my little brother complains about not beaing able to hear. So i was planning on adding speakers. Probably a pair of 5x7? Inifinity? will i need to get an amp or will the stock radio be able ot handle it? and how do i got about doing this? will the ssv still work i ove that feature.