Most of the suicide bombers in Iraq are foreigners. The volunteers are numerous, but they come prepared to die. The Sunni Arab Iraqi antigovernment organizations that come across these foreigners, pass them on to al Qaeda groups, who get the volunteer ready for the mission. Sunni Arab groups have been helping with getting cars (bought or stolen) and equipping them with bombs (usually artillery and mortar shells wired to explode when the driver pushes a button.) But most of the suicide car bombs have been al Qaeda operations. Few Iraqis have volunteered to be suicide bombers, but the concept is popular in other Arab countries, where Palestinian suicide bombers have been turned into folk heroes. Many of the volunteers dont want to kill Iraqis. These are often told to go home. Others are convinced that they will be killing Kurds (who arent Arabs, and are ethnically related to Iranians, who are much hated by Arabs) or Shia Arabs (al Qaeda is a Sunni movement that preaches death to Shia for not being Sunni enough.) Some of the suicide volunteers, the ones who arent too bright to begin with, are simply deceived and sent out on their mission. Its not like the guy is likely to come back and complain that he was tricked.
The foreign volunteers are eager to kill coalition, especially American, troops. Some of the suicide car bombers are still directed against American troops, and sometimes they succeed. But most of the time they either cant get into position, or American troops shoot them. So the volunteers are given secondary targets, and these are the ones that are usually hit. The volunteers drive off with a non-suicidal guide/minder, who plays navigator until they are within sight of a target. The guide then arms the explosives, bales, and the volunteers drives off to do his best.
There have been 100-150 suicide car bomb attacks so far, with many more aborted, or the drivers arrested or killed before they could set off their explosives. Over 500 people, mostly Iraqis, have been killed by suicide bomb attacks so far. The attacks have made al Qaeda, foreign volunteers and Sunni Arab rebels very unpopular with most Iraqis. This is what al Qaeda wants (the better to start a Sunni/Shia civil war), although it is not exactly working out according to plan. Over a third of the Iraqi dead are Sunni Arabs, and Shia Arabs and Kurds are increasing their own security (with volunteer guards, or simply more civilians willing to point out attackers to police or coalition troops.) This forces the suicide bombers to increasingly hit targets in Sunni Arab neighborhoods. This is one of the reasons there have been so many attacks on police stations in Sunni Arab areas. While this demoralizes the police, it infuriates the Sunni Arabs because of all the Sunni Arabs killed in these attacks.
Seven suicide car workshops were found in Fallujah, and several more have been found in and around Baghdad. There are obviously more out there, and they will only be found when enough Sunni Arabs get fed up with the bombings and let the police know where the workshops are.