Need some help please...

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Okay... I am trying to hook up a single 12" sub in my Yaris sedan. I have hooked up many systems before in my previous cars BUT, they were aftermaket head units and ran the RCA cables directly to the HU. Now heres my problem/question-

I want to use my stock HU and run a Scosche SLC4 converter (low quality I know, but I'm just looking for a lil more thump than the stock speakers) to send the signal to my amp. I have everything hooked up correctly as far as the sub and amp goes.. but I dont want to tap into the back of the HU wires to get the front speaker signals. Can I just run the converter from the rear
6x9's ? And if so, do I still need to run 2 sets or RCA's? Anyone use a similar setup? If so how did you hook it up? Here are some pics of some of the hardware I'm trying to setup. Any help at all would be greatly appreciated!