My old guitar amp doesn’t have a line in jack to allow me to mix my guitar signal with an audio source as a backing track. To remedy this, I created my own mixer. More details here: http://www.ssguitar.com/index.php?topic=4028
The short of it:
1 x small wooden box
1 x 1/8″ stereo jack
1 x 6′ stereo 1/8″ cable cut in half
4 x 200 ohm fire-resistant 1w resistors (1/8w would have worked and they don’t need to be fire-resistant, but it’s what I had)
For each channel coming from each device you put a resistor in line with the positive. So Device One’s left positive goes into a 200 ohm resistor. Device Two (amp) has it’s left positive go into another 200 ohm resistor. Both of these resistors terminate at the left positive of the headphone out jack. Do the same with the two right channels, tie the grounds all together and you’re done!