don't know that anything will go wrong. But where are you gonna get the power to run the sub? I am taking it that your 6x9's are powered by your h/u. that isn't enuff to power the sub. You'll have to and should run the wiring like we all have to.
i didnt run the wiring i directly put it to 1 of my speaker in the bak and the sub is working but not like its supposed to obviuosly, i just want to know if anything will go wrong if i do that, i dont want to drag any wires because im getting a new head unit.
1st u have to realize that underpowering a sub is just as harmful as overpowering 1. You are gonna have to run wires sooner or later, new h/u or not. U might as well get your amp and run the wires. In the mean time get the sub disconnected from the 6x9's.