Here are a couple of reasons for wanting to hook up your guitar to a stereo: playing a guitar with stereo pickups through a mono guitar amp; playing an acoustic guitar with a high-end transducer system through an electric guitar amp, the quality just does not match. You can wire into your stereo, but the wiring limits freedom to move and dealing with connections can damage the speakers, especially the tweeters. One way to do it safely and play through any stereo or radio is with an FM tuner, just as an MP3 player works.
Plug in an adapter to your guitar. If you have a guitar with a stereo 1/4-inch output jack, plug a 3.5 mm-to-1/4 inch stereo plug adapter into the guitar jack or the output of the stereo digital interface you are using with your guitar pickup system. If you are using a guitar with a standard tip-ring 1/4-inch mono jack, use an 1/8-inch stereo jack-to-1/4 inch mono plug adapter in place of the stereo plug adapter.
Plug in the 3.5 mm mini-plug of the FM-transmitter MP3 adapter to the plug adapter and turn on the transmitter, the guitar and the sound system.
Tune the FM transmitter and the FM stereo/car stereo/portable radio to the same unused frequency and set the volume at a safe level for your sound system.
Adapter plugs can be purchased at most electronic stores.
Turn down the volume before plugging in to prevent pop noise from damaging speakers.