The so-called Syrian Observatory for Human Rights said the Thursday blast killed 43 people and wounded more than 80.
Injured Syrians taken to hospitals in Turkey and later died are among the 43 killed, said Rami Abdurrahman, the head of the Observatory.
The group relies on a network of activists on the ground.
The Local Coordination Committees, another activist group, also reported the car bombing but said only that it killed and wounded “dozens of people.”
Car bombings have become common in Syria as the influence of terrorist groups has risen.
Al-Qaida-linked militants, who are engaged in deadly fighting between rival terrorist factions in Syria, have been blamed for Thursday's blast.
An amateur video posted online showed women, men and children at the scene of the blast near the Bab al-Salameh border crossing in the northern province of Aleppo. In another video posted online by activists, burned out cars are seen in the area near the crossing and the site of the attack, as people walk pass pools of blood, with clothes and other personal belongings scattered all around.
People cross the border at Bab al-Salameh on foot so the garage was filled with vehicles transporting people to or from the crossing.
“Oh God, may you punish them!'' a man shouted as people used fire extinguishers to put out flames consuming two vehicles.
In Turkey, a government official speaking on condition of anonymity because he wasn't authorized to release the information, said 94 wounded Syrians were brought across the border for treatment and that 14 of them had died.
Militants fighting the Syrian government captured the border crossing on the Syrian side in July 2012, opening a key transit point for people and supplies. But the area has seen an uptick in clashes and attacks between rebel groups fighting for control of the crossing in recent months.
The Syrian conflict started as largely peaceful protests against President Bashar al-Assad's rule and turned into civil war after some opposition supporters took up arms to fight the government.
Activists say more than 150,000 people have been killed and millions have been driven from their homes, seeking shelter in neighboring countries and in safer parts of Syria.
BA/BA