Please read my license notice before copying my photos.
I was shopping for powered speakers for my iPod and discovered that everything was either too clunky or too expensive. What I really wanted was something like a 1960s transistor radio. So I bought one, for $3 from the local Kiwanis thrift shop, and replaced the headphone jack with a line input jack. I already had the input jack in my junk box so total cost was $3.
Older technology tends to be much more hackable than newer. Transistor radios have the RF/IF stages completely separate from the audio, and the connection between the two is made at line level through the volume control. So this project didn't even require any modification to the circuit board. There was even a handy wire going from the IF demodulator to the input side of the volume control. I simply snipped this wire and connected it through the line-in jack.
In the photos, the two green wires bring line-level audio and ground from the output of the IF demodulator to the line-in jack. When there is no plug in the jack the audio is switched to the volume control and the radio operates normally. With a plug in the jack, the left and right inputs each go through a 100 ohm resistor before being combined, then passed to the volume control.
Sound quality is about what you'd expect, but that can be a plus when listening to 60s Brit-pop or surf.