Becky Watts, 16, was last seen on the morning of February 19 when she left her house in Crown Hill in the St George area of the city.
The teenager took her phone, laptop and tablet computer but did not tell friends or family where she was going, and didn't take extra clothing.
A huge search has been carried out since she was reported missing by her family at 4pm on February 20.
Local churches have told Premier some of the congregation have been helping search public areas.
Nothing has been found so far but police arrested two people over the weekend.
They remain in custody after detectives were granted an extra 36 hours to question them.
The local church, St Ambrose, has held a prayer service for Becky.
Officers are now appealing for help on a black Vauxhall Zafira car.
Police want to hear from anyone who has seen the car, reg HY06 HYA, between the morning of February 19 until February 23.
An Avon and Somerset Police spokesman said: "The critical time period is from the morning of Thursday February 19 until Monday February 23.
"We are, however, keen to speak to anyone who saw this vehicle in the days either side of this.
"We also need to speak to any previous owners of this vehicle. If you have owned this car in the last few years, we need to speak to you as a matter of urgency.
"We have seized this car but need the public's help in piecing together where it might have been."