Background

Several people took the time to document their work online, and it is their experiences and ideas that allowed me to develop my own plan. Although each person's modification is a little different, all take advantage of one or both of the existing external inputs to the radio: the cd-changer input and the telephone input (which is used by GM's On-Star system in my car).

We'd all love to use the cd-changer input since it's made for playing music, but the radio won't switch to it unless you connect a Saab cd-changer, or a microprocessor that mimics one (see the late, great, tragic device created by Carl Levine at Granite Embedded). So, unless we're very good detectives with microprocessor experience, that leaves us with the telephone input.

The first modification I read about was Seth Evans' (website no longer online, but see David Muir's page for archived copies). He found that it was easy to force the radio to play audio from the car's telephone input because, unlike the cd-changer input, all you need to do is connect a special "tel-mute" connection to the electrical ground. He opened up the radio and soldered a 1/8inch jack directly to the telephone inputs, and a switch between the ground and the "tel-mute" connection. Success!

The downside of this was that the telephone inputs don't produce full-range sound (apparently it sounds a like the sound is going over a telephone connection, with little bass), it's not stereo, the radio's tone controls are ignored, and in the end somehow the wiring short circuited inside Seth's radio and killed the unit!

Showing excellent character, Seth tried again with a new radio. He found a way to make all the same connections outside of the radio using a handy wiring harness in the passenger foot well. In some cars this connector has all of the telephone inputs and logic you need, and Saab will even sell you a wiring harness for it. I believe in my car this wiring harness is connected to the On-Star system.

David Muir has written a wonderful guide for a similar modification based on Seth's work, and he's given us lots of details and background information. I really owe him a great deal for organizing this information and presenting it so clearly.

I wanted to have better quality sound than it seemed you could get with the telephone connection, so I was really excited when I read what German Alvarez had done. He realized that we could bypass the low-fidelity, mono telephone input circuitry and use the cd-changer input to begin with. The trick was to connect the higher quality audio signal to the radio's telephone circuits at the last possible moment. He did this by opening up the radio, cutting a few circuit board traces, and making two tiny connections. This disables the cd-changer inputs, and the tone controls are ignored since the radio still thinks it's in telephone mode, but now the sound is high-fidelity and stereo! German created wonderful little circuits that complement an iPod car adapter and automatically switch from iPod to phone calls--which allows him to keep the telephone function.

So from all of this, I developed my own plan...