Maybe the cops found the key with the car and took time to clean it up (since the only dna was Stevens from what I remember) and planted it afterwards? I did miss some of am episode or two as I had to pop out while my wife was watching so maybe I'm confused. either way I really can't imagine Steven had anything to do with it, just too many inconsistencies and clear fuckery from the police.
That could quite possibly be right. Now that I remember, Sgt. Coburn had that dispatch call where he read off the plate number of the RAV4 and he guessed the year and make of the car after dispatch said who it was registered to even though he denies being in the presence of it during that call. The call was also two days before the vehicle was even found. If he wasn't in front of it, why is he calling dispatch for a plate check? For all we know, he could've found the vehicle with the body nearby and the sheriffs could have burnt the body themselves with the plan to plant it all in the Avery scrapyard.