I see questions on this topic all the time: installing an aftermarket head unit in the IS300. There are many options, but it seems that many questions still remain. I plan to give an overview of some of the options available to you and hopefully clear up any lingering confusion. This won’t be a step-by-step guide as much as comprehensive information about the methods available to you. My advice is to pick the one that you feel most comfortable with and best suits your goals.
1. Probably the most well-known solution is the Metra Tyto-01 harness, which connects your new headunit to the stock amp, allowing you to keep all 4 OEM speakers in place (though you can upgrade the speakers at the same time, if you wish). Many people opt for this method because it doesn’t require a new amp, it doesn’t require 2 new rear speakers, and it can be done fairly quickly. In summary: the RCAs come out of the headunit, plug into the Tyto box, and then a harness comes off of that and plugs into the stock harness feeding in from behind the glove box. You have to take the blue (in most cases it’s blue, but I can’t speak for every single stereo system) illumination wire off the new headunit and splice it into the illumination wire on the IS harness. Don’t forget the ground, or else your stereo won’t work properly and you’ll have to pull it all apart again.
Personally, I don’t quite understand the hype about this option because most have the intention of keeping the OEM system. If you plan on adding a real amp and new speakers, my opinion is that option #2 (which I will get to in just a sec) is easier, but we all have our preferences. The main reason I don’t like the Tyto option is because it involves keeping your stock amp (and in many cases, the OEM speakers), which to me is the weakest link of the OEM system. Most people go through all this and come out with really no improvement to sound quality, which seems to be more work than it’s worth. But, it makes for a simple install of your new and modernized headunit, which for some people is enough. For those of you going this route: don’t overthink it. It’s designed to be simple and quick.
2. Another option you have to renovate your sound system, though it requires a bit more hands-on work, is a separate Metra harness, the 70-8116. This harness will allow you to eliminate the stock amp (and preferably add one of your own, one that can actually push some decent power) while still retaining the stock speaker harness. I recommend installing a new headunit as well; I don’t see the point in going through all the work only to keep the OEM unit. This is important: it requires new rear speakers or you could fry your new headunit/amp. There are probably multiple ways to carry this out, but I opted to run power from the battery (with inline fuse) to the headunit & a switched power wire to the illumination wire on the headunit (you will need this or it won’t turn on); dual RCA cables, activation signal, & single power cable to my amp (grounded the amp in the trunk); and the Metra harness from the amp back into the stock speaker harness location (behind the glove box where the amp used to be). Since the stock rear speakers have four wires feeding into them, you’ll either have to slightly repin the Metra harness or pay attention to what wires are “hot” in the rear and which are not. Wire in the speakers accordingly.
I chose this method for two reasons: 1) I have a 2001, so the Tyto harness is not quite plug-and-play, plus I didn’t feel the Tyto setup was sufficient for what I wanted; 2) The stock amp does not allow the speakers to come anywhere near their potential, and once you put actual power to them you’ll see that they are indeed decent speakers. Even if you run your front speakers straight off the new headunit (without a new amp), you’ll notice that they will sound much much better than they ever have. I installed new 6x9s in the rear, and I will eventually upgrade the two fronts, but for now I’m pretty satisfied.
3. Your last option, which is the most labor-intensive and will turn some people away, is to run your own wires from headunit to amp to speakers and subwoofer (if applicable). For the audiophiles out there, this is the best option to achieve optimum sound quality, as you’ll be able to upgrade to a thicker gauge wire. While it might require the most elbow grease, it’s theoretically the simplest way. You’ll need power and ground coming off the battery (with an inline fuse) to your new headunit; you’ll likely have to run the activation signal from HU to amp; you’ll have to route the RCAs to your amp (presumably located in the trunk; amp will need its own power and ground as well), and lastly you’ll run wires from the amp to your speakers and subwoofer. No Metra harnesses, no nothing. You have a blank canvas to do and create as you please. FYI: Crutchfield has a great chart to help you guys determine what gauge wire you’ll need for your power goals right here.
So there you have it. If you’re looking to install a new headunit (and more) into your IS300, here are your options. Hopefully, I’ve eliminated some of the lingering confusion and misinformation that litters the interwebs. Have fun and go nuts.
Note: the first two options do not include directions on adding a subwoofer; if you are wiring everything yourself, this doesn’t matter.