Intermittent CEL and unusual radio illumination behavior

[cartwheels]

I'm wondering if I have a grounding issue on my 91 SS. When I hit the brakes with the headlights off, the illumination on my radio (stock headunit) dims briefly, and then comes back to normal brightness. If I'm on the brakes and I have a turn signal flashing, the brightness of the radio flashes in time with the turn signal flasher.

Additionally, I have a CEL for the canister purge valve solenoid. I have observed weird behavior, including the CEL turning on at idle, but just off idle (a small blip of the throttle, even) the CEL will turn off. Most of the time the CEL remains on, but I've seen the CEL behaving as if it's RPM-dependent a few times now.

Does this sound like a grounding issue? Anybody faced anything similar? I know the grounds on these older Subarus are not known for being very effective, especially after over 31 years on the road.

[arse_sidewards]

That sounds like a TEXTBOOK ground issue.

The ground is not able to pass sufficient amperage without voltage drop across it. So if there's 0v on the chassis and 3v on the harness (25% voltage drop) then all those things trying to send amperage through that ground are effectively running at 9v hence the dimming.