Looks like I have some cracks developing in the door sills. Anyone had anything similar? I want to know if it's cosmetic or structural. Can I just grind over the top of the seam then put a bead of weld along the seam?

 

Is there evidence of a rear quarter panel replacement?

 

Your car has had both quarters and/or rockers replaced. Body shop didn't seal their joints properly (Many don't, because they don't think the customer will have the car long enough for this to happen). I wouldn't start picking at it, it's worse under the surface. Especially on the passenger side.

 

do the quarter panels have matching vins? Or no vins at all?

 

One of my old s2000's had very similar cracks. Both quarter panels were original including to the paint. The car had somewhat higher miles driven mostly in NYC, perhaps a combination of high miles and rough roads cause them as well.

 

I have seen this happen before on cars that are starting to rust out.

 

Just saw in your other thread that this car is a 99 thats in the UK. It's possible that what you're seeing is due to age and moisture, but those seams don't look right to me. They look too flat, like a prior repair was done.