Electrical Issues Leading to a Slow Death

I started the wagon up earlier and noticed that when my blower motor was on, my tach would hit zero. Same would happen when I turned headlights on. So I gimped her a few miles through town while the sun was going down with no headlights. My dashlights got dimmer and I got a CEL but no battery light. Lost a lot of power and finally started to sputter, so I pulled into a parking lot and walked to work, an auto parts place.

 

I swapped out my 6 year old battery, which fixed everything and let me get the extra 2 miles home without incident.

 

The alternator is 6 days old, so I don't believe it to likely bad, but still possible.

 

I'll check the alternator and old battery tomorrow at work, along with the CEL. My question for now is if you guys know of anything else I should check or watch out for?

Wiring. Look for breaks, corrosion, bad grounds, etc. Go deep in if you have to & inspect the whole harness.