W140 Mercedes S420 Bose Aftermarket Radio Install 

--SEE UPDATE BELOW--
How to video of an aftermarket radio installation on a 1999 Mercedes Benz S420 with the Bose sound system.
Install does not damage any existing harness and utilizes the factory Bose amplifier.
Comment any questions.
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--UPDATE--
System still works great, however the volume is very sensitive compared to the original radio. This is because the aftermarket head units are sending a much stronger/louder signal to the amplifier compared to the original unit. For example, the new head unit is capable of a Max volume level of 50. Level 15 is very loud and I would damage the speakers if I turn it to 50, plus it would sound bad. (amp in the trunk is being told to send too much power to the speakers).
The fix is easy, either don't worry about it and be a little more careful with the volume knob (i.e. turn it up to what sounds good to you). Only downside to that is the volume will have bigger adjustments in sound per click of the knob. Or do as below...
If this happens to you and/or is an annoyance that you want to fix, you will need to install a 4 channel PAC High Low Converter between the head unit and harness. They are 30-50 dollars. You can follow instructions that come with it. In short, connect your head unit harness to the PAC unit the same way I show here (color to color), and then do the same on the other end to the aftermarket harness that we modified ourselves. The PAC unit then has adjustment knobs that you can use to turn the volume down... It nerfs down the signal that the head unit is putting out so that we can utilize more of the volume range. Keep in mind the speakers will only go so loud regardless of this addition. You will only have more range and smaller adjustments to the volume per click on the knob.
Have a good one!
--END OF UPDATE--