Try Looking Behind the pass. side fog light
I bought my "03 SD back in November. I didn't know mine had a block heater until the temps began to drop for real back in December (single digits). Fellow SD owners here in Wichita Falls told me my truck had one, I just had to find it.
I looked behind the bumper by the passenger side driving light. There it was, zip tied to the wiring harness. The power cord was complete and ready for unleashing. After some scraped knuckles and some choice words, I had the cord exposed to the outside world where I could get some use out of it.
I now use it EVERY day. The warmup is SO much faster by using the block heater.
Take a look where I did. You might want to wear some work gloves and send the kids inside the house.
My truck was used when I got it, but 4000 miles later, I'm happy as can be with my first diesel.