Sub or amp dying?

I'm trying to determine the most likely failing component. - sub, amp, or maybe signal from head unit?

2003 Expy Eddie Bauer with full factory system, 6-disc in dash changer. I've had this truck 20 years and never had an issue with the audio until recently.

A couple months back, the subwoofer didn't work after I muted the radio at my work's guard shack. I parked, did my 9 hours, and it worked at the end of the day... that was weird, so I considered it a one time glitch.

Yesterday it didn't work again. I had multiple stops, on/off cycles, going to 6 or 7 different stores. After a couple stops, the sub would notably bump or pop when the vehicle's power system would cycle on or off with the key. Turning the radio on/off or muting had no effect. At on point, I thought it was kind of working depending on the note frequency, but not at all to the level normally expected.

This morning, it worked normal again. When it's working, the factory system is good enough for me. Without the sub, the audio is flat and will be unacceptable. Even though it would sound better overall, I'd like to avoid a costly full system replacement.

On the way home today, the sub sound was fading in and out. It didn't depend on speed, throttle, steering wheel turn.... nothing. Yesterday was hot, this morning was cool (80 for Florida in July) this afternoon was in between. Maybe heat related?... Amp failure based on heat? Who has a thought for me to chase?

No thoughts, but I'm on the same hunt as you. Just yesterday (4th of July) I noticed I wasn't getting any sound from my 2nd row and front speakers. The sound I was getting was muffled and towards the back. At first I thought, well time to fix my stereo but about 5 minutes into the trip everything started working again, all speakers. Parked my 2005 expedition, started it again later in the day and same symptoms.

From my less than 24 hours of research online, I'm also thinking it's a failing amplifier. However, my question is does the 2005 Ford expedition have 2 or more amps? I know my Expedition (Limited with DVD player no navigation) has a subwoofer with an amp but is there another amp and if so, is that combined with the head unit? I'm likely going to replace the head unit with OEM again but would prefer not to throw parts at the problem, though this seems the likely culprit since everytime I have this issue the rear speakers always work whereas it's the front and 2nd row speakers that seem to have issues.

So, this is gonna sound strange, but I may have tracked my issue down, but it's super random. Took the family on a summer road trip and the same symptoms had surfaced this past weekend. Basically, no sound from any of the speakers except the subwoofer and it was super muffled. I've got one of those phone mount/chargers that conveniently attach to the 12v/cig lighter in the front dash lower and she was adjusting the mount to fit her phone when all of a sudden the sound came back! I haven't dug into it, but the issue and potential solution happened a couple of times during our past trip so I know with near 100% confidence it's a wiring issue with the cig/12V plug.

Not having opened up this area, I find it strange that a wire would even exist around this plug that's associated with the stereo wiring but since they're in the same general area I feel like may be using the phone mount on this adapter has caused it to come loose thus affecting other wiring like those connected with the stereo?

I ended up getting help and solution in this thread: https://www.expeditionforum.com/threads/sub-or-amp-dying-thoughts.56291/
My sub amp had failed internally.

After going through my issue, I'd say anything is possible for yours. It sounds like you've identified a starting point for investigating. I'd definitely dig into that area of the power port, check line voltage and grounding, and start following wires to look for chafing or loose wires.