Much to my dismay, the stock radio in my 2007 Prius did not include an AUX in port for hooking up an iPod. Searching around, it turns out that the 51824 in my car does have a CD changer port, and does have the AUX pins, however they don’t function. I even bought an adapter on eBay to use the CD changer port with an external source, and the stereo didn’t even take any notice of the adapter. I sent it back, and instead opted to replace the whole radio with a 51824, which does support these features.
I forgot to take photos of the wiring while doing the install, so here is the only photo I could find illustrating which pins are important for AUX input. The plug that is on the car’s side was missing the wire receivers, so I found some which were about the right size and pushed them into the plug with the stereo already plugged in. You can put a switch between pin 19 to 16 if you wish to selectively disable AUX input, but I didn’t care for that, so I wired them together.
I found a long 1/8 male to 1/8 male wire, which worked well as a donor to this project. I cut off one end and wired the cable’s red wire to the stereo’s pin 15, white to 17, ground to 16. I fed the cable around the dash’s framework and zip-tied it along the way so the wires don’t get pulled from the stereo. Reassemble your car’s interior, and if all goes well, you should be able to cycle through your stereo’s sources, AM->FM1->FM2->CD->AUX (or something similar).
This was a very cheap and relatively quick mod to a what would have been a stock stereo. Assuming your stereo accepts aux input, this is the best option in my opinion. FM modulators are just awful, take the time to do it right! :)