Detectives said two men approached the woman once she had left the vehicle in Cheltenham Gardens, Skircoat, on December 15.
One of the men is reported to have sexually assaulted her, while the second man stood nearby.
The assault took place between between 1.25am and 1.50am, Calderdale CID said.
Det Insp Gary Stephenson said: “We have been carrying out extensive enquiries into this incident and are now appealing for anyone with information to get in contact with officers.
“I would appeal for anyone who saw two men matching the descriptions in the Coronation Road, Cheltenham Gardens, Abbey Walk South, Huddersfield Road and the surrounding streets between 1am and 2am.
The first man is described as Asian, around 5ft 10in to 6ft, and had a stubbly beard. He was wearing blue jeans and a beanie hat, possibly red with a white trim.
The second man is also described as Asian, in his late 20s, around 5ft 8in to 5ft 10in and had a black beard.
Det Insp Stephenson said: “I appreciate this incident may cause some concerns in the community so we have had extra patrol officers in the area to offer reassurance.”
Anyone with information is asked to contact Halifax CID on 101, quoting crime reference 13180627158. Information can also be passed anonymously to independent charity Crimestoppers on 0800 555 111.