My passenger side door has a crack at the top rear corner, about 3/8" long. It looks like a fatigue crack, especially considering the location. I don't know how long it's been there, but I guess it's slowly growing. The driver side door may or may not be developing a crack, I can't be sure even with a magnifying glass, but there is a tiny spot there in the same location.

The car has about 96,000 miles.

The real questions are:
1) Why
2) How to stop the growth and repair the crack.

 

It is from the car flexing. ( sub frame connecters can prevent that from happening and will stop that from coming back once fixed properly. )

I had one on each side.

Just before paint he fixed them the same way he shaved the antenna.

He removed some material from the crack making it a little wider and smooth.

Then he placed a thin metal plate behind it and ran a weld bead over it.

Smoothed the weld down and the crack disappeared "totally"

 

Exactly what Ron said.

Subframe connectors will prevent it from happening in the future after you fix it.

 

Will bolt-in subframe connectors be good enough for less than extreme use? Which ones are the best bang for the buck, and not so hard to install?

 

NOPE, bolt holes will elongate after a while, better to install the weld
in type SFC's . Shouldn't cost too much to have someone weld them in, just make sure you remove your carpet first.

 

I understand.

Once properly done, will that be enough to stop the crack, and repair it with normal fill and paint?

 

I would get it welded, and then smooth it over with filler. Even with SFCs, these cars flex a little, enough to crack the filler easily.