Denver's accident report is online and goes into some detail about the fuel tank selector switch location.
The original Burt Rutan design called for the selector to be located on the floor, between the pilot's legs. It's not unusual for the selector switch to be located inside the cockpit in an easily accessible location, so that, in the unlikely event that the pilot has run one tank dry, he can easily switch to another tank that still has fuel.
The builder of that particular aircraft (kit-built) decided he didn't like the fuel lines coming into the cockpit, so he changed the location to the wing root, outside the cockpit. To this, he added a long torque rod, going through a couple of U-joints, which entered into the cockpit, as you note, over, and behind the pilot's left shoulder.
Because of the complicated linkage, there was binding which made it difficult for the pilot to select between tanks. To overcome this binding, a pair of Vice-Grips (yes) was clamped onto the torque-rod. This was the condition of the aircraft when Denver test-flew it, bought it, then flew touch-and-goes to his death. He had been briefed on how to operate the selector switch -- on the ground. He had never attempted to switch tanks in the air.
The problem was that when the pilot attempted to access the selector Vice-Grip, he tended to step on the right rudder pedal -- hard.
It is a common practice for pilots to 'stick' the fuel tanks prior to flight to precisely determine the actual amount of fuel. The process is exactly as it sounds. You remove the fuel cap, then use a graduated wooden stick to measure how much fuel the tanks contain.
Denver never sticked his tanks. He made an assumption about how much fuel were in the tanks. Denver had been practicing touch-and-goes. On one take-off, one tank ran dry. He tried to switch tanks, stepped on the right rudder pedal, sending the airplane out of control.
Here's the sad part of the story. About a year or so before the accident, Denver got popped for DUI. Per FAA regs, pilots who get a DUI cannot fly. A pilot's license consists of two parts, the airman certificate and the medical certificate. Both are required to fly. When a pilot gets a DUI, their medical is revoked, so they are not legal to fly.
The FAA notified Denver via registered mail a couple of times to surrender his medical. He ignored them and continued to fly. He was illegal when he was flying the accident aircraft. Had he surrendered his medical and stopped flying, he would not have had the accident.
IMO, he kept flying because, in his mind, he thought "I'm John Denver and the rules don't apply to me."
Just like Harrison Ford.
I didn't realize Denver had two hours or whatever in the plane when this happened. Was also not aware of the tendency to push the rudder while you were trying to switch tanks. My buddy has an RV-9 (two seat side by side) and the selector is on centerline in front of and just below the edge of the seat. Much easier location to reach and also no goofy linkage to have to work against. Seems like a much better plan. In his case the valve is actually in the cockpit iirc, but you could probably remote it to below the floor if you wanted to and leave the handle where it is. And he didn't stick the tanks. What a doofus.