Advice on audio setup (spkrs, amp, sub)

[BruthaMan]

Hey guys,

Background:

I have a Retrosound model one receiver. It has four RCA outputs for speakers going to an amp. It does NOT have a subwoofer out. The model two receivers do, but the model one (which I have) does not.

Currently I have two 3.5s in the dash, in the original speaker mounts for a 72 Chevy truck and two 6.5s in kick panels. No amp currently.

What I want to do:

I want to add two 6x9s to the doors. Before anyone breaks out the holy water and exorcism rituals, the previous owner had cut the doors for 6x9s. I've had them covered up with a carpet panel, but since they are already cut, might as well use the space.

I also want to add a four channel amp. Plan to run the 3.5s and 6.5s in a series for the front two channels and the 6x9s on the rear amp channels.

All that seems logical and easy to accomplish.

Here's the kicker (no pun intended):

I want to run a powered low profile subwoofer. I have a Boss Audio powered sub in my 78 and for the music I listen to (mostly classic rock), it's plenty.

So my question is, how can I get a signal to the powered subwoofer without a subwoofer out on the deck? It seems like that signal should come from the deck itself, rather than from the amp?

Appreciate any advice.