First, regardless of where the cars were built, they would be built to the same specs (in all respects) without regard to plant location.
The rear panel color is something that we normally didn't worry about back then (when the cars were new and we were in high school). Worried more about engines, interiors, disc brakes, etc.
The 300s usually had only THREE choices of wheel. The standard "deluxe wheel covers" with the 300 logo in the middle, the Magnum 500 14" wheels (usually 14x6), and the wheel covers for disc brake cars (the ONLY 15" wheels and wheel covers).
The rear panel color HAD to be spec'd at the time the order was made. In a Google image search, I looked for similarities. Like keyed to the color of the vinyl top/convertible top (which could also relate to interior color), color of the car, or an equipment situation (possible "TNT" option, as '70 Camaro factory big blocks had black rear taillight panels rather than body color). I found no such correlations with the 300s, from what the images would indicate. 1967 300s all appeared to be body color, as the '68 Newports were. SO
as this item seems to be a "specifiable" item, the 1968 Chrysler Dealer Order Guide with Color and Trim would be the ONLY real way to see what the real criteria might have been . . . follow the interior color, accent/contrast to the body color, or whatever. If none were specified (as some dealers might have chosen to not do), there had to have been a default mode (which could have been "silver". Black would have been a "no-cost option", as a result. In any event, when the Sales Order was submitted, it all had to have been spec'd at that point in time.
To be fair, most pictures were more concerned with the beauty of the 300s and were usually side pictures or frontal pictures, with only a few of the tail light area. At this point in time, looking for the usual visual clues as to TNT could be suspect too, as adding dual exhausts or changing underhood clues could all have happened after the car was originally built by Chrysler. ONLY the VIN might yield that information.
IF you can find an online 1968 Chrysler Parts Manual, there might be some clues hidden in the part number listings. Plus it would have a full decoding of the option codes and other stampings on the Data Plate, which might have information on a "rear panel color" paint code, in addition to the normal "upper" and "lower" paint codes, etc. Although I doubt that any 300s might have been built with "two-tone paint", as vinyl roofs had become popular and "upscale" and would have effectively substituted for such things, back then.
ONE OTHER THING I found during these searches as an image of part of the 1968 Chrysler wiring harness!
www.diagramonwiring.blogspot.ca Downloadable! In color! Check it out, if desired.
CBODY67
