This is an early truck with the large underhood distribution box not the later solid state type turn signals I think?
Its not good to draw more off one battery than the other by tapping one for 12 volts...the states of charge between the 2 will get farther apart until one goes bad...unless you charge the 2 batteries seperately on a regular basis.
Wire 24 is: Light switch to blackout tail lamp
Wire 490 is a short wire spliced to wire #24 and runs to the trailer receptacle for the blackout tail lamp.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!