Basically, one of the previous owners of my current GSX decided for some reason to bypass the ACC switch, so that the radio, cigarette lighter, etc. would stay on.

Does anyone know the reason behind this, as well as the easiest fix? I have no intentions of using the cigarette lighter, but I have to constantly unplug my radio harness after rides, and have been hesitant to hook up my sub because if I leave it on it would surely drain my battery.

I've looked through the electrical manual, and haven't found too much in regards to it, I might end up opening the steering wheel wiring compartment and looking for evidence of hacking and modification.

Any help is appreciated

You'll have to figure out where that circuit is shorted to +12v and fix it. I have no idea why anyone would want to do this (people perform all manner of horrific mods to these cars in the name of pretend car audio), but I know that there is a common mod (that I have performed myself before) to run the switched 12v supply to the radio from the 12v supply for the windows. This mod is performed at the relay box above the clutch pedal. The window switches continue to get power for 20 seconds after the key is removed so the radio will stay on until you open the door. Check that that isn't the actual behavior first, since the extra 20 seconds wouldn't drain your battery or have any other harmful effects.

If it's truly just shorted straight to 12v and never turns off, you have to start thinking like the idiot that would do this. Where is the easiest place to access both switched 12v and constant 12v? I'd start with the radio harness. Check your radio adapter harness as well as the dash harness for the radio to see if anything has been modified. You can remove the center console and see if the cigarette lighter harness has been tampered with as well. The ignition module in the steering column isn't the most difficult to get to but there are easier places to get the job done so I'd look in those places first. If you don't see anything anywhere else then you can take out the panel under the driver side dash as well as the column trim pieces and check the connector that plugs into the back of the ignition tumbler.

Follow your hot wire back and see if it feeds all of your components. If it does, just unhook it and rewire it to its perspective fuse/location.

Check in the fuse box left side of the foot pedal, hope they did just a jump to the fuse.

The radio actually has two (2) power feeds, right into its harness plug: One is switched 12v, and the other is constant 12v, used for keeping the clock circuit running (assuming that radio has one).

It may be as simple as the PO bridged the constant 12v to the switched 12v lead. Check the radio harness and make sure there are no jumpers installed. A wiring diagram can identify the correct colors for you to check.

Errr...there is a power button on the radio. Either labeled source, or the circle with the line through the middle. Hold it for like 3 secs, should shut off, till you fix the wiring at least.
I am assuming it's an aftermarket radio, if they used a quick disconnect aftermarket harness (White plug that goes into your factory wiring that connects to the radio harness) Yellow is 12v constant, and Red is 12v switched. If he wired the red and yellow on back of the radio to the yellow on the harness, just take the red off and put it to the red on the aftermarket harness.
Odds are he wanted to listen to music while the key was out, and charge his phone or other accessories.

Hack is putting it lightly . He must have really wanted his narrowband and boost gauge (which didn't work from the day I got the car) working. He stripped the radio, etc. when he sold it, did a battery relocation with what looks like 8 gauge to me (I switched it to 2 Ga), and all of his gauge wires strung around the fuse prongs in the interior fuse box. I have 12V at the cigarette lighter (which he tapped into) and both remote and mem radio harness wires. I know all "On" functions are halted such as windows, ignition, etc, but all ACC functions are functional (head unit and cig lighter).

I even had to jump out a wire from the ICM to the coil pack because he had a wire cut inside the car, causing coils one and four not to fire.

I removed all of his wiring visible to the eye without opening up the steering column, to no avail.

I'm good with wiring diagrams, and electrical as a whole but the mitsu electric manual doesn't go much into the key ignition, so I wasn't sure if anyone had any experience. I certainly don't want to go into my ignition while I need my car at school. I will probably try just turning off the head unit and seeing if it will kill my battery.

Let me know what else you guys got, as I can't see anything else hacked in the center dash. I might actually throw a relay in line with the window wiring as Randman talked about, as currently I can't find any other usable switching sources.

The gauges wired into the fuse box

Still sounds like it's got a switch or wire jumpered to acc from a constant. You need to do more investigating, preferably with a DVOM and flashlight before moving on.

Previous owner tied the radio fuse into the interior headlamp fuse for some reason. Ran into the issue a little down the road when I was going through the fusebox.