Many threads have appeared discussing how to add aux to a base Prius radio. Most approaches involve adding a 3rd party device to the CD changer connector on the back of the radio. As an alternative I opted to upgrade the radio to add aux. My 2007 base Prius comes with factory radio 51830 which does not have aux nor can it play MP3s. I purchased radio model 51824 from a salvage company for just over $100. This is similar to the base radio but has aux and MP3 capability. I used instructions on PriusChat to remove the existing radio which was relatively easy. I discovered the base Prius has no aux wiring whatsoever so this turned out to be a challenge. The first attachment shows a photo of the back of the Prius radio with the 20 pin connector which came with the upgraded radio. The wires are clipped because it was easier for the salvage compary to do this than pull out the connectors. By examining Prius radio wiring diagrams I determined which wires needed to be added for aux (shown in photo). I needed to connect a 3.5mm stereo cable to the 20 pin connector. To do this I used 4 of the pins in the clipped connector which came with the salvage radio. When soldering the pins to the 3.5mm cable, the left audio channel goes to the tip of the 3.5 mm connector; the right audio channel goes to the connector next to the 3.5 mm tip, and audio ground goes furthest from the 3.5mm tip. The aux sense input just needs to be connected to audio ground. The next attachment shows a photo of the 20 pin connector in my Prius after adding the 4 aux pins. Yes its ugly but connections are solid. The wire connecting aux sense to audio ground is very ugly because it was soldered in place in the dash. The last attachment shows how the 3.5mm female connector fits in the tray below the radio. The aux works fine with the salvage radio. A message even pops up on the MFD when aux is connected. This is somewhat surprising because the base Prius MFD does not expect aux to be there. This brings me to the MP3 player. This works perfectly fine except that the MFD does not display any information about the music. There is no Title or Artist nor is there any display of play time. You can advance tracks and folders via the steering wheel controls and using the radio controls. Now for a surprise ending. It turned out the salvage radio had a problem ejecting CDs so I returned it. By the way the company I dealt with was very good about it and even paid to ship it back. Instead of getting another radio I decided to put back the original radio and buy a 3rd party device. I will report on that endeavor soon. Edited to say photo in last attachment was taken after the original radio was put back in the car.