Is there a list of them somewhere?
The grounds you're looking for are not that many, and not hard to locate.
The way I do it is to run the engine and turn on some loads, like the heater fan on high with the headlights and rear window defroster on. Then I use a digital voltmeter with one probe on the negative battery terminal and start touching the other probe to different parts of the engine and chassis. With good bonding, you should never see any voltage difference at all, maybe a tenth of a volt at most. When a ground is compromised, you may see upwards of a volt or more. This is usually accompanied by other symptoms like lights that brighten and dim, radio memories that reset, blowing bulbs, and other untold anomalies.
If it was a ground issue it would not of suddenly appeared after many years and would be likely intermittent too.
When you drive cars old enough to vote, all sorts of strange things happen. I've troubleshot it twice on my own cars and dozens of times when I worked at a 2-way radio shop. On my cars, both times it was due to negative/ground wires that corroded inside the insulation where you couldn't see it, leaving only a few strands to carry the current. No sudden failures, and not intermittent. Just a gradual worsening of symptoms.
Swap out alternator with a rebuilt one.
Years ago when a rebuilt alternator was $75 that might be good advice. I looked up a 2005 rebuilt Focus alternator at NAPA and they're going for about $175. That's an expensive "try it and see". It would only take a few minutes to check the grounds with a voltmeter. That would include the radio grounds too. If the negative power lead for the radio is compromised it could be getting return current through the coax, which can also cause wonky behavior.