I think he may have asked someone in his small circle to move his car every two days, maybe paid them to do so. Like an assistant or a fanboy?
Then it may have become very smelly, and perhaps that person decided to stay away from the car after that, realising something was wrong?
Of course, there is still the possibility that someone in his circle framed him. Maybe there was a love triangle or some weird situation, and someone else harmed Celeste and decided to put the evidence in D4's car?
Even as I write that^^^, it is hard for me to believe it wasn't him, because of two main things----he seemed obsessed with Celeste, and his lyrics and videos were so dark and twisted and focused upon death and murder and bodies in vehicles, etc.
So even though we have to ask "WHY would someone put their murder victim in their own vehicle?' If I look at his songs and videos and his obsessive behaviour, then I could believe that he would do that because he has acted that same vision and scenario out on the world stage for a couple of years already.
