Why on some 1996 Mustangs i've seen the rear light are horizontal? Shouldn't they be vertical for that year?

 

I could be wrong but I thought they changed in 97.

 

I believe Justin is right and they went to the vertical taillights in 96

The saleens however remained horizontal I believe.

Otherwise, the two assemblies are interchangeable so maybe that's why there were different ones there.

 

They did change them to the vertical lights in 96. While they were interchangeable, there is more involved than just swapping the light itself, you have to swap the harness as well, or modify it at the least. My guess is that the cars were wrecked and the cheaper lights were put in, or the person who owned the car just liked the older lights better.

 

Someone mentioned that these could be Japanese Imports?

 

Not sure about Japanese imports. My 96 is a German export car but the lights are vertical. Anything that was slated for overseas had export only tags on parts affected.