bmxer and 12kk. it technically could be plug and play. and bmxer that is the correct location. the non infinity kks do have this subwoofer option after market though.
if drilling into that plastic scares you like it does with me, have a shop fabricate one for you. fiberglass molding is easy. but tuning it. that's a whole other story. yes you could just plop a sub in there with out a chamber, but that would sound like rattling cans...
if you do enough research, doing this "mod" is simple.
what you can do for a good price is get a speaker to rca converter. the first link below is the best for the price. or you could go with a PAC adapter. if you have a 2010 to 2012 kk, the 2nd link below would be perfect. and its a slightly easier install then the lc2i. the lc2i just has more fine-tune control then the PAC.
if you have an 2008 to 2010 KK the PAC AOEM-CHR3 will work. 2011 and 2012 KKs, we don't have any product listed that will work on their site, but the C2A-CHY23 will work. not sure why its not listed on PACs site but is on amazon. buy it before they stop manufacturing it!!!!
what I would recommend: the PAC adapter. mainly because its literly plug and play. prices set aside because they are both at the same price point. I will be installing the PAC in my KK soon. so im not going to recommend something I wouldn't buy.
if ya'll find some other PAC or adapter, pm me and ill let you know if its worth your money.
http://www.amazon.com/AudioControl-2-Channel-Converter-Subwoofer-Control/dp/B004CTBYGE/ref=sr_1_40?s=automotive&ie=UTF8&qid=1424468441&sr=1-40&keywords=speaker+converterhttp://www.amazon.com/AudioControl-2-Channel-Converter-Subwoofer-Control/dp/B004CTBYGE/ref=sr_1_40?s=automotive&ie=UTF8&qid=1424468441&sr=1-40&keywords=speaker+converter