The wiring diagram for 1948-49 in the shop manual reprint shows the connections you have are correct however there should be one more. The wire from the battery terminal of the solonoid should go to the same terminal as the horn wire (the + terminal).

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.