I installed a system on my stock radio before i got a chance to grab a head unit and its easy but dont make this a permanent solution. aftermarket head units play cleaner sound and frequencies plus it gives you highs and lows the stock unit can't play.
First i would get the basics out of the way like running the positive amp wire through the drivers side from the battery to trunk where you will hook up your capicator and amp. you can ground your amp in the trunk right behind the middle piece where the liftback's latch is at.
Now you said you have an Xplod amp as in Sony, so your amp should come with speaker inputs or a speaker input harness (check amps manual) so you dont need the Line Converter, but if you dont have these speaker inputs get the line out converter as mentioned by DisGqBoi.
After this take off your glove box compartment so we can get access to the stock amp (im assuming you have a GTS radio) or take off the center console if you have a GT radio so you can get access to the stock unit. Go to the "Installs" page on this site and print out the stereo color code wiring info so you know what wires to tap into.
Now with the wiring info you got tap into the rear speakers (there will be 4 wires a + and - pair for the right rear and + and - pair for the left rear) for the Line Output Converter you either bought or the Speaker Input on your amp. (whichever you had to use). If you use the line output converter it will give you some RCA outputs so you will run RCA cables from there all the way to the trunk. And for the Speaker inputs on the amp just run four wires from the amp to those wires you have to tap into either at the Stock Amp (GTS) or the headunit (GT).
Almost done... The remote cable is also listed in the Installs page for the GT and GTS so just tap into it and run a wire all the way to your amp and hookin up your subs with your amps are left and i will leave that up to you cause this is long enough..
if you have any questions, ask. ill help out...
hope i helped