Stock amplifier frequency filtered?
 

Is the signal from the stock amplifier to the door woofers crossed over in any manner?

I doubt it is highpassed, but is it lowpassed or are the door speakers left to roll off naturally on the high end frequencies?

I don't know the answer to your question, but I will add that the signal going INTO the amplifier is full range. Also, the signal going to the rear speakers (at the rear speakers) seems to be high-passed.

Hopefully someone can answer the question as to what is coming OUT of the amplifier, then all the info will be in one place!