Unless you guys track down the owner and have him present or have written permission from him, I wouldn't touch it.
Otherwise, technically, he could claim you inflicted more damage on the vehicle, you could be charged with criminal mischief, etc.
If it stays there, and I assume it is on Forest Service land, only the FS can declare it abandoned and order it's removal. But once they do that they are responsible for the vehicle and are charged with the care of it and it's contents.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang