2003 GT speakers

With the Mach 460 system, you've got two choices.

1. Install all new wiring to the speakers when changing the head unit, bypassing the Mach system entirely. (Not hard at all, just time consuming.)

2. Route it through the factory amp using a wiring harness adapter (much easier).

I've done it both ways when working with OEM "high line" systems, with very mixed results.

In my dad's Suburban, going through the factory amp and keeping the factory sup with new speakers and a new head unit sounded AWFUL. He liked it because the highs of John Denver, The Beach Boys, and Johnny Horton came through loud and clear, but that was ALL that was loud and clear. Things sounded much better when I bypassed the amp years later.

In my friend's F150 with the factory Kicker upgrade, replacing the radio, all four door speakers, and the front tweeters resulted in a pretty good sound all around. He bought from me the Infinity Reference 6802cfx speakers I'd run for three years, so they were well broken-in and sounded good from day one for him. (I highly recommend those speakers if you're keeping the amp, I was feeding them with just the head unit, and they were BEGGING for more wattage).

Then there's my 4Runner with its factory 10-speaker JBL Synthesis audio system with a factory-installed DSP, amp, and dual-voice-coil subwoofer. The factory sub was blown out when I bought it, so I replaced it with a Rockford Fosgate Punch unit that was virtually a direct fit. I discovered after the fact that the volume knob on the factory radio didn't turn up the volume directly, but rather controlled the output from the amp, and that the EQ settings were controlled the same way. After installation of the new head unit, it sounded hollow, and I couldn't make it sound good no matter what I did with the new head unit's EQ. I found out what I did wrong this past week and fixed it, and now it sounds amazing.