Its not that hard to skip the mach system. Forget tapping into it, not worth it. The radio isn't good enough in my opinion.
Pull the radio, add a 4channel amp, and run your ampd signal back up to the factory wiring in the dash. The tweeters in the 460 are NOT amplified in the stock mach setup so these are good wires to use in order to get your signal out to the doors without running new wires all the way there. Cut your door speaker wires that come from the factory amp, then run your tweeter wires down to them. Wallah, you've ran your amp out to the factory wiring and then from your factory tweets to your door speakrs. Its probably a good idea to replace the speakers too but you can do that later if you don't have the cash right away. Your only held back by yourself, not the mach system!