You need something like this to make your after market HU work with the Mach 460...
Car Stereo Output Converter
I have a prior version and my Alpine has always worked just fine with the rest of the 460.
The adapter above with RCAs work, but you need to have a hu than has a built in high pass filter for the internal amp because you'll be giving the tweeters bass notes without it. I tried it with my Kenwood KDC-X891 and took it out because I didnt like how it sounded. Although I didnt mess with the settings enough.
I used one of these for my friends 00 GT with the Mach 460 when he had the factory amps. Worked perfectly and sounded fine. The speakers would pop when the HU turned on though without a resister on the remote wire.
You don't need that to make the Mach460 work correctly if you have a HU with sub output and crossover control. All you need is this wiring harness Ford Receiver Wire Harness For select 2003-up Ford, Lincoln, and Mercury models at Crutchfield.com and you just plug the harness with the RCA jacks into the sub output of the deck. The only time you need that is when you need to convert speaker level to RCA level which you don't need to do with the Mach460 system.