Just those two vehicles, the rest spawn fine as they were added by the RDE team, which surpasses my knowledge base of anything to do with injecting custom stuff. I added both hwaycar4 + hwaycar5 into the same place, as RDE has their hwaycar's like hwaycar2/3, etc. Naturally I added cars from here to replace those in the hwaycar slots already made by RDE, which those work perfectly fine as they just replace something that already exists. But, I figured I'd try adding more hwaycar's, which seem to have been added fine, but refuse to spawn.
Out of curiosity, what would be the big difference from adding lines from the carvariations in the DLC of RDE? Vs. adding it into the DLC of mpchristmas? They're both DLC folders, does one take precedence over the other?
Yes, RDE is in the DLC list. RDE automatically does that for you when you install RDE via the oiv.