I would try checking the wires on the door side, rather than under the dash.
No experience with this, but I'm right behind you.
My subwoofer will arrive any day now this week.
I will connect it to the rear speakers, just so if I mess up, missing sound from the rear speakers will be less of a hassle than having the front speaker fail.
I suspect that DSP is
Digital Signal Processing, in this case meant to amplify certain frequencies or low frequency response compared to a regular sound signal; or run the audio digitally over wire, instead of analog.
Like, you can't tap a wire off of another wire that's meant to run to a subwoofer.
It runs best when it's ran off of a full range (regular analog) speaker output.
The rear speakers on the Fiesta ST, are regular door speakers, full range. No subwoofers.
The only way your sub can be affected, is if you change the occupancy mode setting from "driver" to "all occupants" (or something), or reverse.
It might affect the sound a bit, but it'll still run fine.
I assume, I can run a wire straight from the rear door speaker, to the subwooferamp under my driver seat.
I won't tap the wire from the location you mention, but will try to solder it straight on the rear door speaker.
You can test the wires by using the radio 'fade' and 'balance' options.
Run the radio on the speaker you will want to tap off only, and then test the signal wires (you can do this with another speaker).
You can also check to see if you can remove the rubber protection hose connecting the body to the door, to see if you can see the wire color.
I haven't tried this method before, but it'd be something I'd try first.