Verify grounds and cables. Test static battery volts. If low, trickle
charge at slow rate overnight. If not yet at 12.6 volts, charge longer.
Disconnect the charger, turn on the high beams and blower motor and watch
the volts drop for 30 seconds to a minute, then shut off everything and
watch the volts rise. Within 3 to 5 minutes it should rebound to 12.6
volts. If it does, the battery is good to go, as long as it's clean. If
not, weakness is showing. If it doesn't make 12.5 within 5 minutes, time to
replace the battery for sure.

Now that the battery and connections are proofed, we can go on to proof the
charging. At idle check voltage at the battery terminals. Now turn on the
high beams and the blower motor/s. If you have 13.5 volts with load at
idle, it's probably not your issue, but that's weak enough to put an
alternator on the budget before it fails. A new alternator will be 14v or
close under load and over 14v without the loads.

I've seen Rabbits get by with 13.3v rates under load, but the Westies and
such can have lots more loads to supply.

Note if you skip that first step you are wasting a lot of time. No
corrosion at all allowed, no quicky terminals, new cables if necessary.
Then make sure the top of the battery is kept clean. This will double the
life of the battery, alternator and starter, and prevent excess life
sucking stresses from killing a new battery/alternator/starter in a half life.