“Grid Kid” murderer John Giuca got a boost from an unlikely source in his bid for a new trial — a retired NYPD detective who had worked the original murder case against him testified on his behalf.

Testifying in Brooklyn Supreme Court on Friday, ex-Detective Thomas Byrnes told a judge he’d had a hard time believing the jailhouse snitch at the center of the 2003 murder of 19-year-old Fairfield University football player Mark Fisher.

The jailhouse snitch, recovering heroin junkie John Avitto, had helped put Giuca away by saying the accused gang boss confessed to him in jail — only to recant this year.

“Did you believe John Avitto?” Giuca’s lawyer, Mark Bederow, asked Byrnes on the witness stand.

“No,” the ex-cop answered. “I did not believe what he was saying. I did not believe parts of it and I thought, when I first met him, he was looking for something.”

Avitto was trying to get a good deal on drug charges he was facing, the cop said.

Prosecutors counter that Avitto’s recantation is fishy and happened only after Giuca’s lover recently cozied up to him.