2000 sterling won’t start

I swapped cab on 2000 Sterling with a C12 cat engine. I bought same year cab. Everything is identical. Truck started and ran fine before the swap. But now when I turned key on and push the start button it won’t start. The first time it made the normal sounds. But as soon as pushed the button it sounded like a relay blowed and killed everything. I was very careful with the harness and fuse panel during the swap. And feel confident that all was hooked back up exactly like before. Any help appreciated

 

Yes I checked all. They all good. I’m wondering if because I had to u hook the trucks computer if it caused a problem?