The battery is right there in the back under the spare, why wouldn't you just use the best possible power source when it is so close to where you're amp will be?

Stock wiring is miniature compared to what you would want to supply a 200w RMS amp with. This wattage @ 12V will be pulling 15-16 amperes, through your fuse box! Not a good idea... get a few feet of 8-10 ga wire and use a fuse holder no more than 12 inches from the battery.

Don't waste hundreds of dollars on audio gear and then use it to potentially damage your X!