KANSAS CITY, Mo. – Only a handful of Los Angeles Chargers dressed quietly Thursday night as rap music thumped through the cramped visitor’s locker room at Arrowhead Stadium.

Some of the 40-plus men in the room bobbed their heads to the beat, while most of the others smiled and laughed as they changed clothes.

Some yelled enthusiastically as they wove between a mass of bodies to the showers and back to their narrow stalls, all the while reliving a monster come-from-behind 29-28 win over the Kansas City Chiefs in front of a national audience.

“F – – – overtime!” one Charger yelled to his nearby teammates, who jubilantly cheered the nod to coach Anthony Lynn’s decision to go for two (and the win) in the waning seconds of the contest.

Only feet away, rookie safety Derwin James, an emerging star, held court with a group of reporters, explaining what the Chargers proved to themselves after improving to 11-3 and into a tie with the Chiefs for the AFC West lead.

“We’re here — we’re here!” James said. “This ain’t the same old Chargers. We’re a different team. We’re here, we’ll compete with anybody in this league, and we showed it tonight.”

It’s hard to blame the Chargers for being so fired up. They had lost nine straight to the Chiefs, dating back to 2014, and appeared to be headed toward loss No. 10 when they fell behind 14 points in the first quarter.

And when the Chiefs scored a touchdown to take another 14-point lead with about eight minutes left in the game, it looked like another bad loss in Kansas City was brewing for the not-quite-ready-for-prime time Chargers in the sea of red that was their annual house of horrors.

Only, it wasn’t. And a big reason for that was quarterback Philip Rivers.

Rivers threw two interceptions Thursday, continuing his maddening trend of turning the ball over against Kansas City. He threw 14 during the Chargers’ nine-game losing streak to the Chiefs, and Chiefs fans let him hear it after his rough start.

But when it mattered most Thursday, Rivers delivered. After the Chargers got the ball back down two scores, Rivers completed 12 of 18 passes for 132 yards, a tally that included a few outrageous darts and floaters, not to mention a touchdown and the game-winning two-point conversion to receiver Mike Williams, who was left wide open due to blown coverage.

“We did not come here to tie,” said Lynn, who eschewed the extra point for the game-winning two-point conversion. “We came here to win. To me, it was a no-brainer.”

Especially if you have faith in your quarterback, as Lynn clearly does. And the moment Williams hauled in Rivers’ pass and the Chargers rushed the field, left tackle Russell Okung made a point to seek out his quarterback, who teammates describe as a man who holds himself and them to a high standard but is beloved nonetheless.

“I told him he’s the best — he’s the best I ever played with,” Okung told Yahoo Sports, while smiling at the memory. “I told him he’s a mother-effing killer; he’s cold as ice.”

Rivers smiled — “He gave me a little bit of a wink,” Okung added — before James joined the cacophony of teammates who sought out the 37-year-old gunslinger.

“I told him I love him,” James told Yahoo Sports. “I’m happy as hell for Philip, man. He genuinely cares about all of us, and we’ve got his back.”

In professional football, the power of belief is a real thing. And while the locker room scene proved the Chargers currently have it, down the hall there was a group that has to find it again.

The Chiefs’ locker room emptied quickly Thursday, long before reporters were let in, and the few who remained spoke in low, monosyllabic tones. It was a scene eerily reminiscent of Kansas City’s recent playoff failures, with the small group of players left to explain another defensive collapse in a big game often repeating the need to “finish.”

And while some Chiefs fans will complain about the officiating — there were dubious calls down the stretch, including a pass interference penalty on Kendall Fuller that set up the final touchdown — it’s the defense’s inability to close out games that will give their fans nightmares going forward. As it should, by the way.

For the first time in weeks, there was no magical throw from Patrick Mahomes, no Houdini act … and that’s OK, considering he’s 23 years old. And so is the fact coach Andy Reid’s generally peerless offensive play-calling sputtered, resulting in a late three-and-out that gave the Chargers the ball back with a little over two minutes left.

Between the two of them, Reid and Mahomes have more than carried their weight this season, and football is a team game. So when the final showdown came down to Rivers vs. Chiefs coordinator Bob Sutton and his 30th-ranked defense, it would have been nice if that unit made a stand, one that could give fans a reason to believe. Instead the Chiefs failed, and now they’re left searching for answers as a tough road test looms against a surging Seattle team that is difficult to beat at home.

“It’s gonna force us to look in the mirror, this late in the season with the playoffs coming up,” safety Eric Berry told Yahoo Sports.

If the defense can’t do it, that will add to the already-considerable load Mahomes, an MVP candidate, is forced to carry … which he might be up for. After preventing the Chiefs’ seemingly annual Bad Omen December Loss last week against Baltimore, teammates said he handled the frustration of losing Thursday night as well as can be expected.

“Pat is ice cold,” linebacker Reggie Ragland told Yahoo Sports.

And once Reid briefly spoke to the team in the locker room about the need to finish, Mahomes not only mentioned the importance of using this loss as fuel for the rest of the season, but also remembering they can still earn the AFC’s top seed by winning out.

“It’s pretty easy [to believe in] him,” right tackle Mitchell Schwartz told Yahoo Sports. “It was never forced … it’s pretty natural.”

So if you’re looking for a reason to believe the Chiefs can bounce back, remember that belief in an “ice cold” quarterback is a powerful thing in the NFL. The jubilant team down the hall was proof of that.

The Chargers believed in Rivers on Thursday, and were rewarded with a win that not only clinched a playoff berth, but also set them up for more, including the No. 1 seed and perhaps most importantly, a sense of redemption against their longtime torturer.

“We needed to beat these guys,” a happy Rivers said afterward. “It’s been a long time.”

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