The appointment follows the recent acquisition of the dealership, which was previously known as Direct Car Finance, by the Auto Credit Trust Group in July this year.
It also marks the second time that the Leeds-based media independent Brilliant Media has lost the business in the same year. In February, before the takeover, it reviewed out of Brilliant and appointed Red Cell to handle its creative and MindShare to handle media following a full-scale review.
However, just two months later and after a series of creative disagreements with Red Cell, Direct Car Finance parted company with the two WPP companies and moved its business back to Leeds. It was taken over by Auto Credit Trust three months later.
Approved Car Finance is a specialist motor dealer selling new and used cars to people who have previously had problems getting credit. It was established in 1996 and has turnover of £150 million from its network of 16 dealerships in the UK and Ireland.
The combination of Approved Car Finance and Auto Credit Trust created one of Europe's biggest motor finance businesses, worth around £300 million.
Walker Media has been tasked with driving efficiencies in the responsiveness of the company's print-based and TV advertising.
Richard Whitton, the managing director of Auto Credit Trust, said: "The media acts as a commercial lever. It needs extraordinarily high levels of thought and energy applied to it. Approved Car Finance is intent on rigorously improving this critical area of our business. We feel Walker Media possesses all the attributes needed to formulate an effective campaign."
Christine Walker, a partner at Walker Media, said: "Approved Car Finance is looking to improve its return on investment through media and this plays to our strengths as a company, demanding absolute attention to detail in both the planning and buying disciplines."