First, I would suggest you go to the PAC site and download an A.E.A. diagram. While the print quality is not the best as far as identifying wire color labels, the wire paths are OK and more importantly shown going point to point rather than using Packards pick a wire number and find it again system.
https://www.packardclub.org/wiring/1942wiring-su8cyl.JPG If you mean the rear tail lights not getting voltage, those are protected by a fuse. If you look at your factory diagram you will see 8A coming from terminal 3 or tail light terminal on the headlight switch. There is an inline fuse before it goes into the loom and comes out 8A at the instrument light switch as feed for that switch. They use the feed terminal at the instrument light switch to branch to several other 8's, one of which is a wire 8B going thru an inline connecter and out of that connector as 8C on to the tail lights.
The parking lights (17) and tail lights (8) are on at the first notch but the second or headlight notch keeps the tail lights but turns off the parking lights anytime the headlights are on. If you had fog lights they would also be fed from the tail light terminal;
9A is coming from one of the auxilliary fuses on the headlight switch and is power for the brake lights.
The factory drawing on this site for the 20th series all models does provide most of the wire size, color, and descriptions of where the various number runs originate and end.
https://packardinfo.com/xoops/html/downloads/wirediagrams/1942%2020th%20Series%20All.pdf On your original problem though, if the turn signal switch and circuitry is not factory I doubt we will be able to help much. There are so many variations of what could have been done and probably for competitive reasons it seems each mfg picked their own solution.
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