The high level inputs on an amp are for funning high level speakers. You want to run subs (low level) Get a line out converter and hook it up. that one you linked to on crutchfeild is a good one cause it has adjustments. You need your input to you amp to be routed via the RCA connections or 99% of amps will not produce anything in the low range.
The converters are easy to hook up, just get to your speaker wires and tap into them. You do lose some power on the stock speakers so make sure you use the rears, with subs on your won't really need the rear except for "fill" sound anyways.
If you got more questions on how to do it right, PM me.