[rexs73gto]

I have a quick question to. I have in my 73 GTO 2 rear speaker now , but originally it was 1 front 1 rear. I have a later model AM/FM stereo now that doesn't use the convector. I have some after market speakers in the rear hooked up to the speaker plugs in in the back of the radio, & don't use the front speaker right now, but it does have a spot for it with the 2 rear speakers. It is a 4 speaker radio from a similar car. The thing I have to ask is about the volume. When I turn it up it starts to distort at a little less then half volume. What can I do to increase the volume as running down the road with the windows down I can just barely hear it . They are just some cheap speaker from I think walmart but the ones that came with it which were the small 5" sound very good in my other car. Is it the radio or ??? Any help would be appreciated.