The '48/'49 Chevy truck wiring diagram I just pulled from The Old Car Manual Project doesn't show the heater. Must have been an option.
Because of the "2 fan motor" heater/defroster you have I'm guessing your 12 volt conversion wiring system is aftermarket. Don't you have a diagram?
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| 
