The 15 men, believed to be aged between 25 and 40, were found with their hands and feet bound, shot in the head and chest, according to Major Falah al-Mohammedawi of the interior ministry.
The vehicle -- either a pickup truck or a minibus, according to varying reports -- was found on the main road between Amariyah and Ghazaliyah, two mostly Sunni neighbourhoods in west Baghdad.
It was found not far from where a minibus containing 18 bodies was discovered last week.
This comes after the bodies of at least 40 more men were found in various parts of Baghdad, 13 of them in the Sadr City neighbourhood where a series of bombings killed around 50 people a day earlier.
All had been shot and many were also bound by hand and foot.
Three men were found tied to electricity pylons and four others bore signs saying "traitor".
Some of the others had been tortured.
The discovered add weight to growing fears that Sunni-Shiite violence is escalating.
Authorities have refused to identify many of the victims, fearing that will trigger yet more sectarian anger.
A wave of violence swept the country following the bombing on February 22 of a Shiite shrine in Samarra.
Officials admit that over 450 civilians were killed in the fortnight that followed the bombing, while 81 Sunni mosques were attacked.
President warns of war
Iraqi President Jalal Talabani earlier told the nation's feuding political parties to work fast on forming a unity government, saying this week's bombings were meant to "inflame sectarian strife and fan the fires of civil war".
"It is the duty of the political blocs to intensify their efforts to form a government and establish a broad front to achieve security and stability," said Mr Talabani, a Kurd, in a statement.
Politicians said they would step up negotiations but doubted a deal would be possible by the time the parliament elected in December met for its first session on Thursday.