WARNING: THIS MOD IS CURRENTLY UNDER CONSTRUCTION. PLEASE READ THREAD BEFORE ATTEMPTING.
This DIY was designed using a single DIN premium Monsoon stereo. It should be possible with any of the MK4 OEM VW head units but you will have to locate the tape deck preamp chip yourself using the images I’ve supplied below. Of course, with any other head unit there may be additional steps or changes you will have to make as you go along.
And now for a disclaimer: following this DIY will be done at YOUR OWN RISK. You are responsible for any damage or loss incurred by attempting this procedure. I assume no responsibility for your mistakes or ineptitude.
OEM stereo from your vehicle
Proper stereo removal keys
Soldering iron (fine tip)
Electrical component solder (silver solder)
Donor cable with connector of choice (with leads stripped and tinned)
Small zip tie
Step 0: Repeat after me: DO NOT USE FORCE. If you are using force, you've done something wrong and it's almost always going to lead to problems.
Step 1: Before beginning any of this, make sure that either (1) you have the radio code to enter after you’ve reinstalled the stereo or (2) you know that taking out and putting the stereo back in will not permanently disable it.
Step 2: Insert the key in the ignition and turn to the accessory position, then to off. This will disable the theft prevention system on the radio. The little red light on the radio should not be blinking when you perform the next step.
Step 3: Insert stereo removal keys (or whatever you have built as substitutes) into the slots shown below and push them in all the way. Pull the keys out and the stereo should come with it. Remove the keys from the stereo by pushing the tabs on the sides and pulling the keys out.
Step 4: Unplug the power (black), speaker out (brown) and multicolor (CD/Monsoon amp if present) connectors from the back by squeezing the tab on the side of each one before pulling it out. Also remove the antenna from its connection.
UPDATE: If you want to leave the tape deck functional skip steps 5 and 6 and just move straight to the soldering. If you don't want to hear the tape you have in there each time you plug in/unplug your ipod continue with these steps to disable the audio input from the tape deck. When your ipod is plugged in the audio from the tape deck will be cut out btw.
Step 5: Bring the stereo to a suitable workbench with good lighting as you’ll need to see very clearly where you’re poking a hot soldering iron. Remove the top of the stereo by prying it up from the back edge. This should expose the inner mechanism of the tape deck unit.
Step 6: Unplug the ribbon which carries the signal from the tape deck head to the circuit board. The ribbon is shown here unplugged already. To do this, grasp the white connector shown in the next picture with a pair of needle nose pliers and rock it side to side until it moves upwards about ¼”. The ribbon should then be able to be freely removed with a gentle tug. Move it into the position shown and replace the top of the case. This will trap it under the edge of the case and prevent it from shorting anything inside the case. If at some point you’d like to listen to a tape, simply replace this tab into its connector. Then ask yourself why you just did that instead of listening via your new aux-in.
Step 7: Turn the unit over and remove the case on the underside. This will expose the main circuit board of the unit. Be very careful about what you touch on this side. Preferably you won’t touch anything besides the component we’re working on. Grounding yourself is not a bad idea if you have a grounding strap.
Step 8: We will be working on the near left corner of the stereo as it is seen in the above picture with the case removed. Make sure you’re looking at the correct part of the board.
Step 9: The next image (taken from inside the stereo which you can’t see) shows the chip that we’ll be tapping into and the corresponding pinout for the chip. This chip takes raw signal coming from the tape deck head and performs preamp, and signal cleanup (among other things) before passing it out as R and L channels. If you’d like to learn more about this chip, just google the part number on the top of it. The image after that shows the chip on the board as you’re seeing it now (from the underside), and the correspondingly flipped pinout diagram. Again make sure you are properly oriented before proceeding.
Step 10: We will solder the right channel in (usually a red wire off of your cable) to pin 16 and the left channel in to pin 2. The ground (usually copper/silver) can go wherever you please that’s grounded, however I found that the ground shown in the picture below was a nice one, as it’s nearby and connected to a mechanical switch which doesn’t mind being heated while you’re soldering, as opposed to surface mounted electrical components which are very sensitive. I took the additional step of insulating the grounding wire with some shrinkwrap to prevent it from shorting anything while it’s in there. I would highly suggest you take similar precautions.
Step 11: Now that you understand where we’ll be soldering, pick up the unit and take off the cassette side cover again. Feed your cable of choice into the back of the case into the slot just next to the antenna connector as shown here from the inside of the case. Thread a zip tie around the bracket shown below and wrap it loosely around the cable. Once you’ve finished the soldering and adjusting of cable slack we’ll tighten this down. The zip tie will prevent any movement or force on the cable from disturbing your tiny solder points.
Step 12: Feed the wire through the case so it comes out as shown in the picture below. Replace the stereo cover and turn the unit back over to begin soldering. Please excuse any continuity issues you may notice with my pictures.
Step 13: Solder the R, L and ground leads onto the pins as discussed above. Make very sure that you use extremely short leads so they don’t contact any other metal and that during this process you do not short any circuit with your leads or solder.
Step 14: Once you have the soldering complete, adjust the cable length so that the covers can both fit and tighten the protective zip tie very snug to isolate the solder points from movement.
Step 15: Replace the covers and return to your car with the completed stereo. Thread the auxiliary input cable wherever you please through the dash.
Step 16: Plug in all of the original cables and turn the key to the accessory position. Power on the stereo; you may have to wait a moment for the car computer to recognize the stereo, or input the radio code at this point. Once the stereo is functioning, put a tape in to get the unit to draw signal from your cable. Side A/B doesn’t matter just as long as the tape is rolling.
Enjoy your new auxiliary input! If you have any questions, comments, corrections or suggestions please don't hesitate to contact me or post here. I'll be updating this DIY with improvements as I make them.
Stay Tuned for DIY Updates:
How to eliminate the need for a tape to be in the unit with the motors running.
How to install an iPod docking cable which will allow you to both charge and play the iPod (12v charge models only).