2009 Ford Taurus - Low Voltage?

So, a few days ago we bought a battery
from WalMart. When we connected it to
the car, the horn started blowing and it
caught fire. The fire was in between the
battery and engine block. Smoke (that
smelled like burned hair) came from the
vents and around the radio. It burnt up the
voltage regulator in the alternator. We
replaced both the battery and alternator
and the voltage is still low. It should idle at
12.2, however it idles at 11.13. When it's
off, the voltage is 11.748. What could be
the problem? We were thinking fusible links
or voltage regulator connector, but I'm
unsure if this car has those things. Again,
the battery and alternator are brand new
and, when you test the computer, no
codes pop up. We are desperate for help!