Mid-to-late 50s?

Rick,

Stuart is correct for the 5-fuse box used through 1949 and the 2-fuse box used for the rest of the Advance-Design years. Most likely the same holds true for 55-60 (mid to late 50s?) trucks.

Here is a 1955-2nd series wiring diagram:
http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm1202.htm
The 55-2nd Shop Manual states that there was also an accessory junction block inside the dash panel.

What year truck are you referring to?

Tim