So I've been trying to figure this out for a few days now. It doesn't look exactly like a quaternion, radeon, or eular measurement. Here's what I think it might be:
WPL (Stream section)
Unknown 4 - quaternion real (expressed in an unusual way?)
Unknown 5 - quaternion j (expressed in an unusual way?)
Unknown 6 - degree of variation
I think Unknown 4 & 5 could be a single axis quaternion set since cars would only need to be rotated along the Z axis (they're kind of rotated then dropped by the game since spawning a vehicle at a higher elevation always results with it being on the next solid object below it).
I believe Unknown 6 has to do with some sort of randomization or variation since I can record the heading angle using the simple native trainer on certain vehicle spawns with the exact same values in the WPL and the heading angles are all very different (within a few degrees =/-)
Any thoughts on this? I'm trying to develop something to take vehicles objects placed via the simple native trainer's object spawner and translate them into something that can be imported into a WPL file. I have everything done except for the rotations. I just have to find how they are expressed in the WPL files so I can setup the translating.