You need several wiring harness to install a Mach 1 sound system in your V6 Mustang. The Mach 1 has amps located in the trunk that your car does not have. I believe the V6 only uses the head unit's built-in amp.

yes the only harness u might need is to hook the speakers up all u have to do for the amp is run ur wiring to it no harness for the amp is required just a remote or (power antennae wire) (blue) from the rear of the head unit to the amp a power wire from the battery to the amp and a ground to be mounted as close to the amp as u can then u need ur rca cables from the back of the head unit then u run from the amp to the speakers ur speaker wire thats it dont listen to the previous comment about u need a whole shit load of harnesses bull shit man just follow my instructions ive installed plenty that guy probably works for circuit city or some shit trying to get u to spend a paycheck or two he is probably a manager

A Mack 1 stock upgraded sound system consists of a series of harnesses that run all over the car. Not to mention that there are two Mach amplifiers that are both mounted to the sub enclosure in the trunk that are also not in the V6 basic system. You can install a Mach 1 system in any mustang that was offered with the Mach 1 option, but you'll need to put all of the necessary hardware in place for it to work. With equipment in hand you can modify a standard system to be a Mach system as you said, but there's more work involved and a bunch of money must be spent on the parts required to do it. Is it easy to do? Yes and no. For someone who knows how to test the inputs and outputs on an OEM amplifier, and adapt factory wiring to provide the proper signal and power, it might be fairly easy. For someone who wants to plug it in and go, without getting too technical, it will be a ton more costly. If the Mach 1 parts are readily available, and not too expensive, it might cost close to the same as an aftermarket system, but it won't sound as good. I'd go aftermarket the whole way if you have the budget. You'll be much happier with the results.