i want to keep my stock HU and install subs into my system.. I already replaced all the speakers and re-ran wire with a 4 channel amp but now i dont know how to hook my 2nd amp up for my subs becaue my 4 cahnnel line out put converter is full... do i need to buy another one b/c there are no pre amp out puts on the stock HU?

 

No, you can buy some "y" spliters and run some new rcas from the rear channel of the line out converter or the 4 channel amp should have 3 pairs of rcas build on it, 2 are inputs and 1 is an output. Take some rcas plug them into the output on the 4 channel amp and run them to the input for your sub amp.