As you can see in the picture I'm missing the back portion of my fog light assembly and the light itself. My question is can I just buy the back portion or do I have to buy the entire light assembly?

The lens is clear still just dirty from the missing back portion.

 

Likely the entire light.

 

you are missing roughly 90% of the fog light anyway, you can probably go to a scrap yard, or ebay and get a good deal.