I have one of those general purpose 12V auto testers. It came with a
tubular incandescent bulb, about the size of a 3AG fuse. The bulb burned
out, and I am not going to waste a lot of time trying to find a
replacement. The whole tester cost about $15.

But before I toss it, I got to thinking that all I really need to do is
open it up and wire a LED inside of it. Pretty basic. I think the LED
should outlast those bulbs too. I have a bunch of LEDs, but none have
the built in resistor.

These are just generic LEDs. I have several colors. None of them have
any specifications. All I know is that they are the indicator type, not
the super brights. I am aware there are all kinds of math formulas to
determine the needed resistor, but since I dont know the LED specs, I
can only take a guess at best.

Knowing this, what value resistor should I use? I'd rather go on the
high side, so the LED dont burn out. If it seems dim, I can lower the
resistor value some. What would be a safe resistance to use?
This will ONLY be used on 12V DC automotive batteries.

I'll clip it together before soldering it on place, to make sure it's
bright enough and working.

Thanks