Both Rhode Island
http://www.riwire.com and Y&Z
http://www.ynzyesterdaysparts.com carry the proper automotive grade plastic wire in numerous colors. Believe both are 10' minimum per color and RI may have the greater assortment. Don't know if RI has a $$ minimum but Y&Z does. RI does not have an online store so you have to write down what you want and call them.
Go to RI's supplies section and the wire is at the bottom of list. At Y&Z check out the online store.
You can download the 54 wiring diagram from the literature section
https://www.packardinfo.com/xoops/html/downloads/wirediagrams/1954%2054th%20Series%20.pdf and it is one of the fortunate few where Packard provided a color and wire gauge list along with the diagram.
I believe the rule of thumb is 6v items have about twice as many amps draw as 12v so wire size on 6v is generally one size larger than used on 12v.
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