I probably should have private messaged you this since it's a bit off topic, but I figured a bunch of people probably have the same question.
I got an iPod/MP3 integration kit from GTA Car Kits. The USB is for iPod/iPhone though, but you can still use the 3.5mm function of the kit for any MP3 or phone you have with music on it if you don't have an iPod. They don't look all that professional but take my word for it, you will love their product. I bought one for my 2005 Camry and absolutely loved it. When I got my 4Runner, I immediately bought another one without hesitation. I had never even heard of this kind of product before until a buddy of mine that I work with told me about it. I always thought you had to buy an aftermarket radio to get MP3 integration. Anyways. It is $99, but in my opinion it is the best 100 bucks I've spent on an aftermarket accessory by far. If you have steering wheel controls, they will work it too (when connected via USB of course). The USB also charges your iPod/iPhone as well. Here is the one you need:
AUX and iPhone Car Kit for Toyota 4Runner 2003-2009 | GTA Car Kits
Basically it's just a box with a special cord that plugs into the CD changer port in the BACK of your factory radio. And yes, your SR5 4Runner does in fact have the PORT available for a CD changer, it's just not actually equipped with one unless it's a limited or whatever. So right now it's just an empty plug in the back of your radio ready to be used.
I hate to sound like a commercial haha. But the best part about these guys is that they have a detailed, step-by-step instruction video that is specific for your year, make and model. That is extremely rare. Don't have any doubts in your head that you can't do the install yourself. Here is that video:
GTA Car Kits - Toyota 4Runner 2003-2009 iPod, iPhone and AUX adapter installation - YouTube
The kits, however, do not come with the in-dash 3.5mm mount that I installed on mine (see pictures below). I bought mine on Amazon for $20 bucks and have also had no problem with that product either. If you do go this route, you'll need a female red/white to male 3.5 adapter to join the in-dash with the kit. Here are these products:
Amazon.com: PANEL MOUNT 3.5MM PLUG: Car Electronics
Amazon.com: HDE ® 3.5mm Male to 2 RCA Left & Right Audio Female Adapter: Electronics
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