"Evil has visited the earth in many forms.....now it returns as 'The Car'. A car possessed. Who knows what it wants. They all know nothing can stop the car. There's nowhere to turn, nowhere to hide, no way to stop 'The Car'. What evil force drives the car." The Car (1977) Official Trailer.
On October 28th 1987 a letter was mailed to the Vallejo Times-Herald stating "This is the Zodiac speaking. I am crackproof. Tell herb caen that I am still here. Tell the blue pigs if want me I will be out driving around on Halloween in my death machine looking for some kiddies to run over. Cars make nice weapons. The pigs can catch me if They can find me out there. Just like in the movie The Car. Tell the kiddies watch before they cross the street on Halloween nite. Tell Tochi my new plans."

It is unknown whether this correspondence is a genuine Zodiac communication, and the content of the letter is vague. However, we will attempt to discover the inspiration behind this correspondence and the location of where the threat was intended or inferred to have been carried out, even though it never materialized. 
The Zodiac Killer mailed the 'Exorcist' letter on January 29th 1974, just over one month after the USA release of 'The Exorcist' movie on December 26th 1973. He then mailed the 'Badlands' letter on May 8th 1974, just over one month after the Los Angeles, California release date of Badlands on March 29th 1974.
​The Car was released on May 13th 1977, so this time the Zodiac (or a hoaxer) would take 10 years to reference this movie in the 1987 letter. Was this really the primary inspiration for this letter, or had the author read something in the newspapers far more recently that triggered his recollection of the 1977 film, bearing in mind the tone of the letter and his threat of running over kids in his vehicle.
Just over a month before the arrival of this correspondence, on September 12th 1987, Jose Santo Bugarin (26) and Rodolfo Alvaro (31) were traveling on King Road, San Jose, California when their car slammed into four children playing on the front lawn of a residence, killing three and injuring one. The driver then backed over the bodies and left the scene. Both were apprehended a short time later. "Bugarin was arrested on three counts of second-degree murder. Alvaro also was arrested on murder charges." Chicago Tribune.  This was covered extensively in the newspapers.

"SAN JOSE (AP) A man who ran over and killed three children entered a plea bargain on Wednesday, pleading no contest to second degree murder and two vehicular manslaughter charges. Jose Santos Bugarin, 26, of San Jose, had two of the original murder charges filed against him changed to manslaughter. He will be sentenced to 21 years and eight months in state prison. Bugarin was the driver of a car that plowed through a front yard on King Road in San Jose on Sept. 12, killing three children." San Bernardino Sun- December 3rd 1987.
​Did the Zodiac Killer or the author of the 1987 letter read the news stories of this senseless crime and callously draw inspiration from this tragedy (Cars make nice weapons), compelling him to write the above correspondence, piggybacking off newspaper articles, in not dissimilar fashion to his early communications. 
The Car movie featured a satanic driverless car running people over in the fictional desert town of Santa Ynez in Utah. IMDB.
However, Santa Ynez is not so fictional in California.
The town of Santa Ynez is one of the communities of the Santa Ynez Valley. It is situated just 20 minutes or 12 miles (as the crow flies) from the site of the Robert Domingos (18) and Linda Edwards (17) murders on June 4th 1963, widely touted as an early Zodiac crime.

Anton LaVey was awarded the title of Technical Advisor in the movies credits, with a quote from his Satanic Bible.

Analysis of the 1986 'Zodiac' letter. 

The Zodiac Killer or a copycat sent a letter to the Vallejo Times-Herald on Wednesday October 28th 1987, claiming he would be driving around on Halloween night in his 'death machine looking for some kiddies to run over. Cars make nice weapons.' He also made reference to the movie The Car, directed by Elliot Silverstein and released in 1977, about a satanic, driverless car that embarks on a series of hit and run deaths. Another review of The Car here. 
The Zodiac crimes involved attacks where he tended to seek out couples in cars, the only deviation being the Presidio slaying of taxi driver Paul Stine on October 11th 1969, yet still a vehicle was integral to the crime.
Quite possibly the Zodiac Killer had used his car as a potential weapon in a previous attack 15 years earlier, when a lone woman in Marin County had a terrifying encounter one dark night.
Isobel Watson (33) had just got off the bus on the evening of April 7th 1972 and was walking home along Pine Hill Road, Tamalpais Valley, Marin County at 9.00pm when a light coloured car veered towards her knocking her to the ground. [view on google maps] The driver exited his vehicle offering to take her home, but after she had refused his offer of apparent help a second time, the concerned citizen suddenly transformed into a rage pulling out a knife and stabbing the woman in the neck and shoulder, fortunately she survived the horrific ordeal. See Tamalpais Valley Attack .  

Francisco, California, close to the Presidio Heights district where Paul Stine was killed, but the building was later demolished in 2001.
It was also exactly six months later that Cheri Jo Bates was murdered outside the Riverside City College Library on October 30th 1966. The San Francisco Chronicle and Los Angeles Times ran articles on LaVey calling him 'The Black Pope'.
Anton LaVey, knew the Tamalpais Valley area well, having attended Tamalpais High School in Mill Valley until the age of 16.
He died on October 29th 1997, but curiously his death certificate stated his passing on Halloween morning. 
Was the Zodiac a integral member or part influenced by the Anton LaVey movement and the press coverage.
In a television interview Paul Avery, who had become the subject of the killers Halloween Card, received a phone call from LaVey stating that he thought the Zodiac Killer may have been one of his parishioners, subsequently providing Paul Avery with some material, leading Avery to comically allude to the fact, that the 'Zodiac was so bad even the Church of Satan didn't want him'. 
As stated earlier the Zodiac mentioned the 1977 film The Car, about a satanically possessed vehicle that embarks on a murderous rampage, Anton LaVey was awarded  the title of Technical Advisor in the movies credits, with a quote from his Satanic Bible.
The murder of Shannon Olson is still highly unlikely a Zodiac crime, with many people suggesting the BTK Killer, Dennis Rader as the possible murderer, as his crimes were in the area of Wichita, Kansas between 1974 and 1991, where he claimed 10 victims, but the majority of his murders involved strangulation or suffocation, only one of his victims Kathryn Bright had been stabbed, once in the back and the abdomen. Dennis Rader has not confessed to the murder of Shannon Olson and the case remains unsolved to this day. 

Mary Apanasewicz of the Vallejo Times Herald claimed then they had never yet received a hoax letter claiming to be from from the ZodiacKiller, however this may have changed all that.
SEE ALSO 1986 LETTER below postmarked March or May 6 / 8th 1986 (although unclear)