By Marty Graham

SAN DIEGO, California (Reuters) - The Mexican help wanted ads offer a quick $500 for a simple job - drive a car into California on an errand for an "important business" organization.

But the new boss may be a drug cartel and the cargo may not be vital papers, or even money, but illegal narcotics. Hidden in the car could be marijuana, cocaine and methamphetamines that, if found by law enforcement, could land the driver in prison for many years.

The drug traffickers' ruse has snared more than 40 people since the start of last year, arrested as they cross the border at the two bustling ports of entry from the industrial powerhouse of Tijuana, Mexico.

The unwitting drug mules tell investigators on both sides of the border the same story: They responded to ads in Tijuana and were simply doing their new employer's bidding.

"They are hiring these people for supposedly legal work as couriers, in sales, vehicle delivery and currency exchange houses," said Alfredo Arenas, of the State Preventative Police in Mexico's northern Baja California.

"When they cross over the border, (the vehicles) are loaded with drugs," he added.

The cartels' new trick was first spotted by U.S. Immigration and Customs Enforcement agents early last year after a run of drug seizures at the ports of entry at San Ysidro and Otay Mesa. The ads seeking drivers were appearing as late as last week.

"In one case, a man we caught told us his wife insisted that he search the car before he crossed the border, but he didn't search the gas tank and that's where the smugglers had hidden the narcotics," U.S. Immigration and Customs Supervisor Lester Hayes said.

"There's often a willful blindness. People who know it doesn't sound right but they figure the less they know the better," he added.

The powerful Mexican cartels based in Tijuana attempt to smuggle billions of dollars worth of drugs each year through ports of entry, clandestine tunnels, and by sea, using ever changing wiles to try to confound U.S. border police.

'SMUGGLERS TRAP'

In some cases the frightened drivers said they were promised a job would be waiting for them in California if they drove the car there from Mexico, said Millie Jones, Special Agent in Charge at the Immigration and Customs Enforcement agency. Once they arrived in the United States they were told they did not get the job and were paid to leave the car.

Offering jobs to bilingual men and women with U.S. passports or visas is just the latest way Mexican cartels use unsuspecting border crossers to ferry narcotics to U.S. markets.

Last year, federal police in El Paso, Texas, uncovered a cartel ring that made duplicate keys for the vehicles of commuters who frequently crossed north from Ciudad Juarez, Mexico, to work or study in the West Texas city.

Unknown to the owners, cartel operators had unlocked the car trunks, often while the owners slept, and loaded them with sacks of marijuana which they planned to remove once the car was parked stateside.

That scheme lead to the arrest of one accused trafficker.

In a bid to curb the new trend snaring drivers using the Tijuana-San Diego corridor, investigators have taken the unusual step of placing Spanish advertisements in the same Tijuana newspapers in which the smugglers advertise.

"Warning! Drug traffickers are advertising jobs for drivers to cross to the United States. Don't be a victim of the smugglers' trap," reads one the advertisements, which the Immigration and Customs agency is running for 30 days at a cost of $2,000.

Besides warning potential recruits that if the job sounds too good to be true, it probably means trouble, the ads include a toll-free number and website information as federal agents try to track down the smugglers behind the recruiting efforts.

"We'll pay for information about how they're doing this and we're hoping to hear from people who didn't take the job because it didn't sound right," said Lauren Mack, a spokeswoman for the U.S. immigration and customs agency.

(Additional reporting by Lizbeth Diaz in Mexico City; Editing by Tim Gaynor and Jackie Frank)