***I have never taken out a Mach 460 stero so I'm going off the other several dozen steros I have installed myself***
1) If your going to use an amp, do not use the speaker wire that connects to the rear speakers themselves. Use the preouts in the back of the head unit. Just use basic RCA cables and run them to the amp. If you splice into the speaker wire, you will lose a lot of sound because the electricity is being split.
2) Also, on the back of your stero there should be a cable that is just hanging out there (if you don't have a stock amp). This is your remote wire. Whenever your stero turns on, this turns on your amp so that the amp is not on all the time. Typically in after market steros, this wire is blue. Might not be in the same in your case.
3) Any clean, unpainted surface on the chasis. You can slide the wire between the head of a body bolt and the chasis itself. If there is paint, just scrape some off so that you will get good clean contact.
If you have any further questions, or would like a drawing let me know and I can help you out further...
