I took a long vacation from Car Audio, now I can't seem to get things to work.....

I have been on an extended break from installing car audio for nearly 10 years..... Children and new jobs and moving..... Now, I finally have some time and the facilities to do some work and I can't seem to get things to work. I have a 2010 Expedition with the Navigation factory system. I am trying to start the install process out by adding an amp and subs. I am attempting to use a Dayton DSP 408 to control the signal. So far I am coming up all zeros..... I have tapped almost every wire in the vehicle trying to find a speaker level to use on the DSP. I have consulted all the information I can find and still can't seem to get anything to go. I have tapped at the factory amp, behind the factory radio, and at the sub amp. All coming up with nothing. I am trying to get the signal to a couple of old Infinity 611a amps and a couple of Ascendant Audio 12's. This is the first stage in trying to revamp the Expedition. I can't tell if I can't figure out the DSP or not. Parts Express was very helpful, I just don't know what is going on.
If anyone has any ideas or anything that can help, I would be very grateful. I am in Eastern Iowa as well, if anyone wants to drop by and help.

find a known good source, like a working door speaker to make sure your gear and wiring process work

I do not mean to sound rude, but if you are using the output from the amp to speakers (hi-level), are you connecting those outputs to the pigtail (not RCA) inputs on your DSP? If so, did you set your DSP to use the hi-level as input?

I'm not familiar with your specific system, but it's likely that you won't be able to use the output from the HU to the amp as an input to your DSP without some type of digital adapter. I have a Bose system in my car and I can't do anything without an adapter until after the amplifier.

Found this, but you may have already tried these connections:

https://www.scosche.com/media/guides/fd1481.pdf

Thank you everyone! Sorry it took a minute for me to reply. Kids, Derecho, Mayor things related to Derecho, and my got monopolized. I was able to get back at this and first off, besides the help you guys gave me, I must say, Parts Express is pretty amazing! I had this unit since Christmas and finally got around to fiddling around with it. They helped trouble shoot and then finally exchanged it at no cost. I am pretty sure everything came back to me, but they followed the old credo about customers being right!! They are amazing, i just wish I had more $ and time to buy more stuff from them to build home speakers and whatnot.

Now, back to this---I have a couple more issues:
1: It all does work, but sometimes the left side of the Expedition cuts out. I then turn off the truck and most times it will all work again. I don't know if the DSP is sending the factory amp into a tizzy. Any advice on that?
2:I fully intend on doing a full build, but right now all I have built is the subs. I am running 2 Infinity Reference 611a into 2 Ascendent Audio 12's. It sounds good, but just no oomph. I have messed around with all the settings, through bluetooth and it still doesn't seem like it is as loud as it should be.
Thanks again for everything!