Fake engine noise has its own speaker which is not connected to the Beats system.
If you install a DSP, amplifiers and speakers, you do not need to code anything, but you need to be careful that you select a DSP with the right functions. Beats AMP has "speaker load sensing", so when you connect the beats amp outputs to the new DSP high inputs (cutting up the harness I linked in the fist post), the DSP needs to mimic the speaker load, otherwise you will get all kind of errors (parking sensors not working, sos call not working etc..)
There is no way to use the "original system" as a audio source to bypass the beats amp. Amp needs to stay in place, and like said:
Speakers outputs from the Beats amp need to be connected to the Hi-inputs of the DSP. That is why the easiest approach is to buy that connector I linked in the first post, so you can cut up the connector wiring, and not the original harness. From the DSP, your run RCA cables to the new amplifiers, and then the outputs of the amplifiers need to be connected to the new wiring harness / connector you previously cut, so the original cabling is now used to "transport" the amplified signal to the speakers.