I currently have a battery light and my battery at idle is at 12.06v and doesn’t change with rpm. my battery is only about 5 months old and the alternator was just replaced about 1000 miles ago. the truck doesn’t die or struggle to start but i’m worried it’s not charging as it should and that it might leave me stranded. I had currently put some 4wd fenders and the truck and moved the air box around I know the alternator wiring runs underneath it. i don’t know if that was the issue but i can’t find any cut, open or loose connecters but im not sure if i moved something the wrong way. I also changed the dash lights and that is when the light came on.

 

If your battery is at 12.06v (at idle or not) you're within minutes of getting stranded. A fully charged battery is usually around 12.2-12.4v (temperature dependent), and a 12.0v battery is almost depleted.

But your problem is probably not the battery, it's the charging. At idle you should have 13.9-15.1v at the battery. http://web.archive.org/web/201102051.../2onvehicl.pdf Your "battery" light is telling you that the alternator is producing no output. As a check, you can measure the voltage to ground at the alternator "A" terminal (the one with the fat wire). If it's the same as battery voltage, your connection is probably good but the alternator is doing nothing.

Your 100-mile-old alternator could be kaput, but first I'd check for voltage to ground on the IG wire (Red) at the alternator. You need battery voltage. If you get less, first check the "Engine" fuse. If the fuse is good, you could have a wiring problem.

Then again, with all of the changes you've made (especially the dash lights), who knows what's going on.

 

Decades ago I acquired a 1987 4R theft recovery. I was putting the dash back together and inadvertantly put a bulb/socket into a spot on the meter that my application didn't call for.
You'd think no harm. no foul, but wrong. It created a few issues. In my case, I put a bulb in the meter where the warning light for"Timing Belt" replacement went. (The diesel truck used the same meter). I forget the other things that got messed up, but it was either charging or oil pressure. But iirc, that was isolated to what the gauges read, not necessarily what was actually going on regarding actual charge, oil pressure.

But that aside, i'd follow Scope103's advice. My gut feel is a defective alternator, but run the tests,
And it's invaluable to start the post with a description of the vehicle. Year, engine etc.