Well, I have validated that the CP-71W does have 4 volts at both the subwoofer and regular low level inputs.

I turned on the original stereo and the original amp played fine through the door speakers. So I will do the four channel conversion of the original amp and hook up the repaired woofers to the subwoofer output of the CP-71W through the separated back channels. The radio supposedly can adjust the subwoofer output +- 10 so that will help on the balancing.

Right now, I am planning on hooking up the door speakers to the front speaker low level RCA jacks via the separated front amp connections on the modified stock amp.

On my previous installation, where I used Kicker component speakers on the back of a base crossfire, I inset the tweeters higher up on the plastic panel. There was some room behind the panel for the driver for the tweeters to fit. I will be looking for some place on the back panel to inset a shallow 3.5" driver (4 ohms) for possible back panel speakers. Looks like I need 1 7/16" depth behind the panel in a 3.5" circle to fit. I won't know until I get the back panel off. If I do find a place, I will hook up the 3.5" driver to the high level back speakers wires of the CP-71W. With the fade and balance controls, I should be able to balance the front and rear speakers. The radio supposedly has 15 watts out and the 3.5" speakers (Kickers) are rated at 25 watts.

This arrangement will allow the stock amp to adjust the sound level based on speed for the woofers and the door speakers (body control module). The back panel speakers will be more of a fill in speaker (per my old Miata), so they won't adjust for speed in this wiring arrangement.

I am still looking at capacitor installation for the front door speakers (installed near the stock amp since everything is open there) and then a capacitor at the new 3.5" speakers on the back panel (if they fit back there). I need to check that the capacitor sizing is not a function of impedance.

I am sticking some Dynamat material in the back cavity and am considering putting the forward facing mounts for the rebuilt woofers.

Also, if there is some kind of voltage mismatch there is a two channel RECOIL line converter that would work on high level or low level outputs for the back woofer.f

If the woofers fail the rebuild, there are some 2 ohm 6.5" Kickers that I can substitute......