1958 and 1959 manuals show a different style lighting switch (from the 1955-2nd switch). All but one 58/59 circuit are protected by two 13-ampere thermal circuit breakers.
1 breaker for the headlamps & parking lamps
AND,
1 breaker for all other lights EXCEPT the instrument panel lights
AND, the instrument panel lights are protected by a fuse at
the rear of the light switch (the clips for that fuse can been seen in that Classic Parts switch).
I have to now go look for the 1956 Truck Shop Manual (which is only a "Supplement" to the 1955-2nd Truck Shop Manual), to see if the 1955-2nd switch or the 1957 and later switch was used in 1956.
It should be noted that if the switch changed between 1956 & 1957, knowing the switch-over date is impossible - GM did not change models on January 1st and did not stop using older parts until they ran out (as long as the older part fit/worked in the place of the newer part.
