Thomas Morphey testified during a pretrial hearing in Will County Court he and Peterson carried a heavy blue container to Peterson's pickup truck in October 2007, the Chicago Tribune reported Friday.
Morphey said he believed the body of Peterson's wife, Stacy, was inside the container.
After the two put the container in the back of Peterson's truck, Morphey testified Peterson told him, "This never happened."
"I won't tell a soul," Morphey said he replied.
Peterson, a former Bolingbrook police officer, is accused of murder in the death of his third wife, Kathleen Salvio.
He is a suspect in the disappearance of Stacy Peterson but has not been charged.
Morphey acknowledged he has been diagnosed with bipolar disorder. He said he has been treated for drug and alcohol abuse.