Well first, never rely on the door ajar light to keep your kids safe. If it burns out, and the door is, in fact, ajar…
Just check and make sure the door’s closed.
Now that you’ve clarified what’s going on, I suspect an answer you got in the other thread is a pretty good possibility – they didn’t properly secure a ground wire after they buttoned everything up. Car electrical systems can do some really bizarre things if they aren’t properly grounded, and if all of that happened at once, that makes me suspicious that it’s a ground.
It could be that they put it back on, but forgot to tighten the bolt down, which would explain why it took a couple of days for the symptoms to show up – it took that long for the vibrations of driving to back the bolt out and make the wire lose contact.
If that’s the case, you should absolutely not have to pay to get that problem cleared up. But please be aware that it could also be that a ground wire just broke because it’s ancient, and it just happened to break within 2 days of service, in which case that’s not their fault and you would be expected to pay for the repair.
Unfortunately when a car gets into the 200,000 mile range, all sorts of things can go wrong with it, and sometimes repairing one thing will put more pressure on the next weak part and break it, and so it can be very difficult to pin a given problem on the last guy who was under the hood.