The link at the bottom of this page points to a radio interview that Donahue did in 1992 via a telephone call to England. There are a few very minor points that I disagree with:
1) Hickey was seated on the top of the rear seat. If the follow-up car was an exact replica of the Presidential limo (with the exception of the running boards on the follow up car and the handles and foot boards on the Presidential limo), then the rear seat was equipped with a hydraulic lift. He was sitting high because the seat was high.
2) Hickey "fell over backwards and by convulsive reflex pulled the trigger." He didn't just fall. The driver of the follow-up car (only five feet behind the Presidential limousine) slammed the brakes in response to Presidential limo driver SA Greer "tapping the brakes to check the tires" when Greer mistook the sound of gunfire for a tire blow-out. Mechanical engineer Dan Roffe (author of JFK Motorcade) determined that inertia caused Hickey to fall towards the front of the follow-up car, and the weight of the AR-15 combined with the inertia, caused the trigger to be pulled. Given the sudden stop and the fact that Hickey was unrestrained, the rifle going off was unavoidable.
3) Donahue's response to the question of "Why was this kept secret? Why hasn't it come out?" was somewhat inadequate. It was the height of the Cold War, and to have it known that Kennedy's death was caused by an accidental shooting by his own bodyguard would have been extremely embarrassing. The Kennedy family didn't want JFK to become a laughing stock, and the government didn't want the USA to appear "weak" or "incompetent." Cover-up was all done in the name of "national security."