wiring question: what is it?

[shifty]

quick question - i'm rewiring my whole truck using the SA-1000 harness from Current Performance and a very small group of wires from my factory wiring harness. I will be deleting the factory bulkhead entirely, I think.

SO. Tell me something - there is a silver box attached to the red and green wires in this picture (bottom middle)....what is it?

Most of my wiring dissection is done - I am actually pretty freaking surprised at how simple the wiring in our trucks is!! There are really only ~2 dozen wires total on a basic truck...a dozen and a half inside the cab and a dozen in the engine bay. Wild stuff to think about how little these things required to run