ok, if i was you...
i'd sell the amp and subs you have now.. use that money to get GOOD mids/highs. kicker, clarion, memphis.. whatever. just do A LOT of research. read a lot of reviews. personally, as mentioned above, i'm a Memphis Audio guy. then buy one of the pioneer 10" or 12" slim line sub and the amp to match it. then build a thin box that's just the size of airspace you need. i would skip on the 4 channel right now if i was you. ONLY reason i have one is b/c i had an old school Clarion processor headunit (didn't put out watts, only volts.. HAD to use an amp w/ it)
what are these 12's you have? and the 1200 watt amp?
alright. let me try to explain it, on my b2. i took Shielded RCA's from the headunit to the 4 channel amp, then ran all new wires from the amp to my mids/highs.
same thing on my mono class d amp for my sub. shielded RCA'S from the headunit to the amp, then heavy gauge speaker wire from the amp to the sub box.
also make sure to run your power wires on one side of the cab (i.e. driver side) and the RCA's on the other side (passenger side). even tho i say use GOOD shielded RCA's, you can still get feedback and whines and noises and stuff sometimes if the power wires are ran over the RCA's.
remember, YOU GET WHAT YOU PAY FOR.
www.crutchfield.com is an awesome website for all car audio stuff; not saying they are the cheapest but there are lots of reviews of most the audio stuff and they have TOP NOTCH customer service.
just always do research before you buy b/c they look good or the specs look good... usually you can find reviews of everything.
and for what its worth-- i would skip the 4 channel amp right now and just run the headunit powering the subs but if your ranger has a factory "amp" you need to run new wires or bypass the factory amp. sometimes its better just to run new wires anyways.
i didn't mean to get so lengthy lol