Thanks everyone for your help. Took it down to AutoZone and they diagnosed the problem as a bad battery. Replaced it and all is well so far.
Seemed like too big of a coincidence for the battery to die the day after I cleaned the posts. Especially considering my head unit is in for servicing so a major power drain is gone... hmmm I wonder if, since the head unit is in for servicing it wasn't able to tell the amp in the trunk to shut off, so the amp drained my battery overnight? And then since my battery is kind of old anyway, it couldn't recover from being drained to such a low level?
I didn't think that was a problem since my head unit has been gone for a couple weeks and I had no problems - BUT disconnecting power to the amp maybe caused it to "forget" it's settings and so it defaulted to "on." Which costs me $60 for a new battery (a bit less than I was expecting honestly!).
