I have the same truck even with the same build month. I will tell you that extra part on the side of the fuse panel is for a timer relay for the fuel tanks. Only installed on the 1980 model year. This truck automatically defaults to the right side tank. The timer relay is there to shut the power off when you get out of the truck and turn the key off so it doesn't drain your battery. So if that relay is missing your left tank most likely doesn't work. I had the same problem. Mine was missing when I bought my truck and I had a hard time even finding a picture of the relay considering its not available through most parts houses. I lucked out and found one in a truck that was being parted out and it works great. They sell them on eBay for 40 to 60 bucks new old stock. The part number is 14014527. Punch that in on eBay and you will find it.
here is a picture.