APOLOGIES IN ADVANCE, this is my first post.
I own a 2005 mercury Montego with about 70k on it. As the original battery started to die i would have to give it a jump to turn on every time i wanted to go somewhere. I got it replaced In January of 2019. After I got it replaced I started noticing my interior lights would flicker and my headlights would flicker. A few weeks after that, my gauges would all drop to zero and my dash lights would go out. I tested the alternator, it tested out at 99%. I wasn’t sure what the problem was but figured I could make it the 2 mile stretch to work and back. Around March I was heading home from work and my car started completely shutting off while driving. My headlights, tail lights, dashboard, radio, power steering, everything would just go out. I managed to drive it home but it would go out every 20 feet or so. I ended up having to pull over and shut the car off for it to work again with no issues. It’s been sitting in my driveway now since March and as of right now, the doors won’t lock, and it again needs a jump to get it to turn on. Any ideas?
OK , I have silly question . Why have you let it just set for almost 4 months ? I assume you have no idea how to fix it and it sounds like a battery or alternator problem so you probably need a shop unless you just want to throw parts at it. You might tell the excellent mechanics on this site how you tested the alternator.
It sounds like the Body Control Module took a crap.
https://moduleexperts.com/5-common-symptoms-failing-body-control-module-bcm/
Tester
The jump starting may have damaged something in the car’s electronics or alternator. Jump starting worked ok on pre-computer era cars, but modern cars with all the electronics, what with the voltage spikes and sparks, not so much. In the future if the battery is dead, recharge it overnight with a battery charger if possible, rather than jump starting.
Here’s the first thing I’d do in that situation. Start with a fully charged battery, terminals & posts cleaned, connectors tightened. Before starting the engine, everything off, measure the battery voltage. Should be about 12.6 volts. Now start the engine. Battery voltage with the engine idling should be 13.5-15.5 volts. Now you have some numbers to go on. What numbers do you get?