Answer: the marketer
Step-by-step explanation:
Based on the scenario given in the question, we can say that General Motors is a marketer.
A marketer is simply refered to as someone or a company that's responsible for researching a particular market and the analysing such market before the creation and exchanging of products.
In this case, we are informed that GM commission McCann Erickson to develop an integrated marketing communications (IMC) campaign using several media in order to reinvent the Cadillac brand, this shows that they're playing the role of a marketer.