The M104 runs each knock sensor into the ECU, they are not wired together.

Have a look at this add on to a MS3, no idea if there is a stand alone unit. ( Also look at MSD as they have lots of ignition stuff ) :

https://www.diyautotune.com/product/ms3-knock-module-kit/.

Product Description

This add on board solders on top of the MS3 daughter board and gives you two channels of knock input that the MS3 can use to retard spark. Using this module will give you a much more sophisticated knock detection circuit than on any previous MegaSquirt knock detection. Features include a programmable bandpass filter, levels that change with RPM, the ability to assign different cylinders to different sensors, and different gain levels for each cylinder. See the MS3 manual for details.

We carried out our own testing with a Bosch 0 261 231 008 sensor – this is a broadband (non-resonant) sensor type that has no built in filtering, so it can be used with a wide variety of bore sizes. Similar Bosch sensors should also work (most of these only differ in their packaging). This knock module can also be used with tuned knock sensors if they are tuned to your engine’s bore size.

This board only works with MS3.

Note: If you are ordering the knock module with an assembled MS3, we can install it in the MS3 for an extra $15 charge. Please make a note of this in the Comments field when you place the order, or this will be shipped as a separate part. However, if you need an assembled MS3 with knock module installed, you may also want to consider the MS3-Pro, as this includes the knock conditioner and real time clock circuit as standard equipment.
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