I still have the original battery (it's a 2004 with near 90k miles) but I highly doubt that is the problem because the battery is still running strong with no side effects. The key batteries themselves are also fine.

I've been putting off this repair for months due to the cost (labour is expensive) and there doesn't seem to be a pinpoint as to what's failed. I think it could be the rear receiver antenna in the glass, but no guarantees. Hopefully I can rectify this over spring break.

The car battery is slated for replacement later this year as well.