Don't use a harness. The MACH 460 requires audio feeds to be sent to the amplifiers first, not to the speakers like a normal stereo. You can try two things:
1) You can TRY splicing into the MACH460s harness manually using all of the audio leads from your head unit. You will need to find some sort of a wiring outline... try the12volt.com... the problem with this is that it is going to be LOUD and may be distorted, because the amplifiers are expecting a pre-amp signal. You will be sending an amplified signal.
2) You can also try finding a harness that works with the earlier MACH sound systems, although I don't believe that one exists for anything earlier than 2000.
edit: come to think of it, my 94 V6 had the MACH system in it, but no CD player. I was able to find a harness for it at Circuit City... I don't know if it would work if you have the stock CD player... I doubt it but you could try..
3) You can get rid of the MACH system altogether and replace everything with aftermarket amps. This is probably the best way and it will sound sooo much better. You will also need a crossover of some sort at every speaker so that the highs go to the tweeters and the mids to the door speakers. So it would be like:
HEAD UNIT PREAMPS --> AMP --> CROSSOVER --> SPEAKER
HEAD UNIT SUB OUT --> AMP --> SUBWOOFER
etc...
4) Last but not least... actually... yeah it is least. You can run wire directly from your head unit to your speakers. This will provide the least power and clarity, but is the cheapest and simplest way to get it working.
Good luck!
-Brian