Taking a quick look at my bookshelves, I got down Patton's biography by Ladislas Farago "Patton, Ordeal and Truimph", Ivan Obolensky Publisher, 1964, 855 pages.
The chapter "And a Time to Die" gives a detailed account. Patton and some comrades are going on a hunting trip. He has already been removed from command of the Third Army by General Eisenhower because of his repeated statements saying the Americans should rearm the Germans, and together both should fight the Soviets.
As they are going down a country lane at about 30 mph, a big truck is coming the other way at about 15 mph, apparently slowing down to turn into a driveway on the left side of the road. It was 11:48 AM on December 9, 1945. There is "litter of the recent war piled everywhere" according to Farago.
Patton, talking with his companions says "How awful war is! Look at all those derelict vehicles, Hap!"
Reacting automatically, Patton's driver, PFC Horace L. Woodring, also looked away from the road. At that moment, T/5 Robert L. Thompson, the truck driver, signaled that he was turning left into the road to his Quartermaster Corps unit.
Patton's driver turned his attention back to the road. Seeing the truck, he jammed on the brakes and turned sharply, as did the truck driver. It appeared to be a minor accident, and all seemed to be unhurt, except Patton. He was thrown forward, then hurled back, breaking his neck and paralyzing him.
He died in the hospital at 5:50 PM on Dec 21st after a series of embolisms affected his heart and he died of acute heart failure.
Another biographer, Charles Whiting, "Patton's Last Battle", Jove Books, 1987 gives pretty much the same details, and then spends 5 pages discussing murder rumors.
He sums up with a quote from a Fred Ayers of the FBI "Logic as well as investigative findings dictate that this must have been an accident.One does not hit a car carrying the intended victim of assasination with a truck at low speed and from an angle. Nor does one normally choose a second vehicle as a murder weapon. It is uncertain, and it may be the murderer who is killed, not his target. Also, I was told that the driver of the truck in question felt such deep remorse that he later attempted suicide."
Yet, the rumors persist to this day.
The Piper Cub incident mentioned in the American Thinker involved a Spitfire flown by an inexperienced Polish volunteer serving with the RAF, April 21, 1945.
There was also the incident May 3, 1945 when an oxcart coming out of a blind lane almost hit Patton's jeep.
I have studied Patton extensively, and I do not believe his death was anything but a tragic accident.