It's in Douglas Brinkley's American History Magazine article,
John Kerry's Final Mission in Vietnam:
As the two enlisted men bundled up Bac She De, Kerry wrote: "I looked at the small green sack that had been booby-trapped, and was awed by visions of the blast that must have ensued when it had been grabbed at. Half a hootch had been blown out by the concussion and there was a frightening hole in the ground." The situation was too dangerous for further on-site reflection, however, and the Navy men started back to their boats. "We had to extricate this body by carrying him in a poncho," Medeiros remembered. "I was carrying the front end, and somebody else was carrying the back. We had to go through a mangrove swamp, tramping through all this mud. I had a [grenade launcher] strung over my back, my radio was inoperable, and we had to deposit this poncho on the stern of the boat."
As the landing party turned back up the path, "light fire suddenly started to rake up from a field over the trees to the left," Kerry recorded. "Everyone dove instantly into the ditch by the side of our dike and I landed in water and mud up to my waist with the muzzle of my M-16 firmly planted in the crap. A whole line of mercenaries had already formed in the ditch, all shooting madly at what seemed like nothing. However, the whiz of the bullets over our heads that was visually nothing was clearly lethal." Kerry continued: "And Bac She De lay in front of us crumpled in the poncho while this holocaust went on. His feet were sticking out of one end, and I couldn't take my eyes off the boots -- one going one way and the other the opposite direction -- and the whole thing just silhouetted where he had been dropped suddenly when the shooting began. The alive shooting over the dead to remain alive.
"I was amazed at how detached I was from the whole scene," Kerry went on. "I just lay in the ditch, not firing because I wanted to save ammo and because I couldn't see what I was firing at, and I thought about what was happening in New York at that very moment, and if people really felt that I was doing something worthwhile while they went down to Schrafft's and had another ice cream sundae, or while some fat little old man who made another million in the past months off defense contracts was charging another $100 call girl to his expense account. And then, when the shooting stopped, I came back to where I was."