Question about speaker outputs on stock, non-JBL system

I am setting up to install a basic, relatively inexpensive aftermarket upgrade. Purchased so far are: infinity Primus 60x4 RMS amp, infinity 6x9 for front door, infinity 6.5" for rear door an REF-375tx tweeters for front dash. I also bought a DS18 V4HL line out converter to keep the stock radio. My question is, how many speaker outputs come with the stock radio? If I wanted to add a small powered subwoofer (say, 8", 100w RMS), would I have enough outputs to easily add the subwoofer? Would a RCA splitter work if needed to "borrow" a signal from one of the DS18 outputs? Thanks for any input..