Head unit modification on a 1994-1998 Saab 900 to add a line input for external media player connectivity.

If you have any kind of suspension modifications, or simply if you drive your car hard, you will eventually get tired of your CD changer skipping on bumps. Worse even, it does the same with stock suspension. An MP3 player sounds like a good alternative, but the stock head unit does not have a line input, so you end up using an FM modulator or a cassette adapter, which inevitably sacrifices the sound quality. And you probably don't want to do that, especially if you just spent a good deal of money upgrading the audio components in the car.

Here's the solution, a relatively easy way to add a direct line input to your stock head unit without sacrificing any existing functionality. There is a couple of alternative ways, this guide shows how to do it by using the tape deck inputs of the internal amplifier.

Thanks to all SaabCentral forum members who did the research and contributed to the installation guide.

24.09.2006
Head unit

Remove the head unit from a car. Use a stereo removal tool, or simply pop the SID and push the stereo from behind. Make sure you have the security code before unplugging the wires!
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Top cover

Start the disassembly by removing the top cover. It's held by two screws (you need a small Philips screwdriver).
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Radiator

Five more screws, and you can take off the radiator in the rear.
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Face panel

The face panel is secured by four latches (two on each side). Unlock those and detach the panel - it's plugged into the connector on the main board.
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Open

Unscrew the top board and pull it up - it's plugged into two small connectors at the rear of the stereo.
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Cassette unit

Take out the screws that hold the cassette mechanism and lift the whole unit (its connector is soldered to the bottom of the small vertical board).
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Rear board

Remove the last screws on the rear of the unit, twist the lock in the middle of the board and remove the main board out of the case.
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Out of the case

The big vertical board can be also detached from the main board. Be careful around electric parts, they don't like static electricity.
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Main board

The underside of the main board has the inputs clearly marked on the tape unit connector. You can tap right here, to the "SIGGND", "L" and "R" (pins 2-4).
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Cassette unit

Alternatively, you can use the tape unit itself - same signals are marked on the upper connector ("G", "L", "R", pins 2-4), and the vertical board offers lots of room and good soldering spots. Take a note of the small switch on the far side, you will need it later.
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Cable

Pull a shielded stereo cable through the holes in the back of the case and install the main board.
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Soldered

Solder the stereo cable to the vertical board on the tape unit and secure the cable with a zip-tie to protect it from vibration. If you're worried about possible interference with the signal from the actual ape unit, you can unplug the tape board from the connector in the upper left corner (see the previous picture), but I haven't noticed any feedback on my unit.
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Circuit cut

Cut the top lead from the switch and solder a wire on each side of the cut. The switch is normally closed (short to ground) when the tape deck is empty and open when a cassette is playing. We want to fool the player into thinking there's a cassette inside.
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Switch

Install the tape deck and the top board back in place. Connect the wires from the cut lead to a momentary switch and fit it between the knobs right above the cassette door. The circuit should be normally closed, and pressing the switch would interrupt it.
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Assembled

Assemble the head unit. You will now have a stereo jack sticking out in the back. Check the cassette door operation - it should activate the hidden switch when you push the door inward.
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Testing

Test the operation in the car. Push the cassette door in to hit the switch. The head unit will load the empty tray and activate the cassette input. You should hear the sound from your MP3 player connected to the stereo jack in the back.
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Installation

Connect the stereo cable and feed it through the centre console into the location of your choice. For instance, there's a gap under the left lower corner of the ashtray, just wide enough to pull the cable through.
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In action

The line input will be always on now, so you can simply switch from the CD or radio to "TAPE" and instantly have your MP3 music on. If you ever need to reset the deck (e.g. to play an actual cassette), just press "eject".

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