i dont know how the mach 460 works or how its wired up but i would think it would be the same as any other car. power wire (red) memory wire(orange) ground wire (black) and then just the speaker wires.
When I replaced the Mach 460 head unit in my '94 GT, all I needed was a wiring harness from Circuit City or wherever and all was well. When replacing it in my '01 GT, I needed a special harness with RCA's on it, which I had to order online. If the '98 is the same as the '01, you can get the hanress anywhere. Hope that helps.