Breakdown

Jeff Taylor and his wife Amy drive cross-country from Boston to San Diego in their new Jeep. Jeff narrowly avoids colliding with a beat-up truck. Later, at a gas station, Earl, the truck's driver, confronts Jeff, exchanging hostile words.
Shortly after the couple resume their journey, their car breaks down on a vacant road. Leaving Jeff with the Jeep, Amy accepts a ride from a passing big rig trucker to get to a nearby diner and call for help. Jeff eventually discovers that someone has tampered with the Jeep's battery connections. After reconnecting them, he drives to the diner, only to discover that no one has seen Amy. When he sees the trucker on the road and forces him to stop, the trucker claims he has never seen Jeff or Amy. Jeff then hails a passing sheriff named Boyd, but a brief search of the truck yields no sign of Amy. The trucker, Red Barr, is let go and Jeff is instructed by Boyd to see his deputy in the town of Brackett.
After speaking with the deputy, Jeff goes back to the diner. Billy, a mentally-impaired mechanic, informs Jeff that Amy left with some men, but refuses to speak with the police, claiming they are involved. Jeff rushes to the location Billy mentioned, but is ambushed on a back road by Earl. He escapes by driving his Jeep into a river, then circles back on foot to watch his attackers salvage the Jeep from the river. He is then discovered and knocked out by Billy.
When Jeff wakes up, he is confronted by Billy (who feigned mental impairment earlier), Earl and another accomplice named Al. Their leader is none other than Red Barr. Red informs Jeff that he wants the $90,000 in Jeff's bank account, and shows him the bank of Brackett. Realizing he has only a fraction of the assumed amount, Jeff attempts to alert the bank manager to his plight. However, paranoid that Red's group is keeping an eye on him, Jeff gives up the idea and steals marked money ribbons and a letter opener. He uses the money ribbons to pack stacks of $1 bills between two $100 bills.
Jeff is then instructed to leave town, where Earl picks him up and binds him with duct tape. Earl begins gloating about how Jeff and his wife were easy targets, how he tampered with their Jeep and his group intends to kill them anyway. Jeff cuts himself free with the letter opener and stabs Earl. After a brief struggle, Jeff takes over the vehicle, binds Earl, and forces him to reveal his rendezvous with Red at a local truck stop. They pass Boyd, who sees the speeding, swerving pickup and stops the vehicle. When Jeff exits the truck with Earl's gun, Boyd mistakes the situation and forces him to lie down. Earl frees himself and shoots Boyd with a concealed gun. Before Earl can shoot Jeff, a wounded Boyd shoots and kills him. Jeff uses Boyd's radio to call for an ambulance and rushes to the truck stop.
At the stop, Jeff sees Red and stows away under his truck. Early the next morning, Red arrives at his farm. Jeff sneaks into the barn, discovering evidence that Red has a history robbing and killing people and that his real name is Warren. Al and Billy arrive with a bound and gagged Amy, and the three lock her in a freezer in the barn's cellar, leaving her to die. Unable to open the cellar door, Jeff finds a gun and demands the cellar key from Red. When he is distracted by Red's son, Billy escapes. Jeff forces Al, Red, his wife, and his son to release Amy, then locks them in the cellar. Jeff and Amy steal a pickup and flee, while Billy returns to free Red and Al, who each pursue the Taylors in their own vehicles.
During the pursuit, Billy is killed when Jeff forces his car off the road. Shortly after, the trailer from Red's truck detaches, causing Al to violently crash into it. Undeterred, Red attempts to force Jeff and Amy's vehicle off a bridge, trapping Amy's leg underneath the dashboard. Jeff rushes out of the vehicle and into Red's big rig, where a struggle over the steering wheel forces Red's truck over the edge and dangle on a steel bridge support. Jeff fights Red on the suspended big rig, eventually hurling him to the rocks below. Jeff frees Amy from the dashboard. Seeing that Red survived the fall, Amy pulls the automatic shifter on their pickup, causing the semi to fall on Red and crush him. Sitting on the edge of the bridge beside their mangled pickup, Jeff and Amy embrace each other.

Question: What component of the Jeep has been tampered with?
Answer: Battery