Where he went through the firewall is probably the easiest, as that's where the clutch mounts on the manual transmission model. This pretty much guarantees a fair amount of room without dangerous bits on either side, and a pretty easy place to drill. However, there are so many good options for wire protection that the failure to use one is terrible. And yes he did recommend using one, but the only way he'll be able to put one on is to completely undo all the wire on the inside and redo it, since he can't get it over the fuse holder.
Firewall Bushings in the Car Audio Department at Parts Express | 425
Personally I figured Kia was smarter than me, so I followed their wires. Stock the main harness goes from the engine compartment, into the fenderwell of the drivers side front fender, then into the side of the dash above the driver's kickpanel. Plenty of room to work, no drilling, and already watertight.
Also, I was able to make a jumper harness using commonly available harness sets (I don't remember which ones they were, but the info is in the giant radio thread of mine) which allowed me to jump into the stock harness behind the radio and just plug in two connectors. No splice, no tap. If'n I was going to try to get the wires somewhere else, I'd probably pull up the front kickplates over the door jamb and see if I could get into the bundle before it goes up the B-pillar. This would let me access the same wires but not have anything exposed.
Finally, (not directed at anyone in particular) don't be afraid to ask for help. There's enough smart people enough places that chances are if you ask, someone will be close enough to help. As an example, I'm in NoVa, and have a pretty nice shop for myself. I'd be happy to help people with their vehicles/stereos. Used to belong to an offroading club and we'd have wrenching weekends, where we'd all go to someone's house and help each other work on their jeeps and whatnot.
Edit: It doesn't bother me enough yet to go out to my car with a multimeter and check, but he does specify what side of the fuse to tap into. If you do such a thing, please make absolutely sure that you're on the load side of the fuse and not the feed side. Tapping into the wrong side is the same as having no fuse. Also remember that fuses are there to protect the wire, not the device. If the turn-on lead for the amp is 16g or larger, it's safe with a 7.5a fuse. If it's an 18 or 20g, it's overfused and could be dangerous. He said it was a scosche fuse, so I assume it was a scosche wiring kit. Scosche doesn't say what size the turn on lead wire is though. There's a reason for doing things the right way even if it's harder, and safety is a big one. Radio amp turn-on leads are usually measured in miliamps, and the wiring for them is usually appropriately sized. Putting a potential 7.5a through that wire could mean a fire, or at least an exciting few minutes. FWIW 7.5a is the size fuse in the heated grips for my motorcycle.