My car turns over, wont stay running, battery is charged, why is it doing this?

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We installed a Kenwood DDX-319 in the car. After we were finished we tried to start it, it turns over & sounds like it is gonna start but it wont. It has backfired a couple times when I try to start it again, it was running normal before this. Could I have done something installing the stereo to cause the car to do this? I have checked the fuses, the volts in the battery are normal, not sure what to look at next. It will not engage and stay running so I cant take it anywhere, Please any and all advice/ideas would be welcome as I am overthinking this and cant come up with anything!! Thanks in advance