You'll be happy with the 5201. And you're right...a backup plan for GPS is not a bad thing. You did pay a little more, but not that much more.
The steering wheel adapter plugs directly into a car connector and will be integrated with the harness that goes into the radio for things like door chimes and push to talk functionality. You shouldn't need the wiring diagram for the car using adapters, but you'll need to carefully match the wires between the adapter and the harness that you've been soldering on. At no point should you modify the car's original harness or wiring, always use adapters and do soldering on them. This should be made pretty easy using the instructions of the radio and the adapter in tandem.
If the kit that you bought doesn't account for the steering wheel controls without modifying the car's wiring then I'd recommend buying another piece that allows that conversion without modification. Not all kits are complete, which is why new cars are such a pain in the butt sometimes. I agree tapping is generally bad unless there's no other way and piercing tap connectors are especially crap. I use plug-in harnesses, solder, and heatshrink when possible.
According to things I've read online, the kit I got is pretty good. I think there are 3 connections. I will have to look. Its the axxess-asw-1. I think that's the model, its off the top of my head.
Yeah, I did some reading on it and it's pretty comprehensive. It seems like you might be able to get away with doing all of your soldering on the radio side of the harness. There's a video on youtube showing a guy that didn't do that, but if there's continuity through the harness there's no reason to tap the car's side of that. The only situation where you might have to modify the car itself is if there's a blank spot on the radio harness's connector so it's not passing the wire you need. In that case I would modify the radio's harness to include the pin in question and connect the adapter's wire to the new pin instead.
Luckily the plug pins are standardized and you should be able to make all of the pieces indoors instead of filling up the car with solder smoke. Since you'll be on the adapter side I'd recommend just using heatshrinkable marine crimps for #16, going direct to a terminal connector on the frame for #15 ground, and adding a pin on the harness for #6 that goes to a soldered and heatshrunk connection with the conversion unit. That's if the car connector looks like the instructions say it should at least. It seems like you know what you're doing, I think you'll be just fine. The hardest part for me is always maintaining patience after devoting a lot of time to getting it right with the details then missing something fundamentally obvious.
As Sierra Nate said, the controls are digital. Most interfaces are boxes that translate the pulses sent from the steering wheel control or SWC which you'll probably see a lot in the manual. The harness from the car will be very populated, I doubt you'll have a single requested pin that doesn't have a corresponding car harness wire. At that point you'll just need to figure out if your stereo harness has the wire needed. During the interface not all wires are carried into the new unit and that's where your potential for lost features is. The adapter that you bought is from a reputable company that's very familiar with car stereo wiring, I suspect that it'll be plug and play minus maybe just those 3 wires you were talking about earlier and they'll be covered in the instructions as to what to do with them.
