WASHINGTON, Sept 30 (Reuters) - Negotiators trying to clinch a Pacific Rim free trade deal made progress toward resolving a key issue on Wednesday when Canada and Mexico signaled a willingness to open the North American auto market to more parts made in Asia, people briefed on the closed-door talks said.

The Trans Pacific Partnership, or TPP, which seeks to cut trade barriers and set common standards among a dozen nations reaching from Japan to Chile, has become snared over a small set of issues, including trade in autos and auto parts, since July.

Officials from Mexico and Canada were aiming for a 45-percent threshold for local content on vehicles, two people briefed on the talks said.

If part of a final trade deal, that would mean the majority of the vehicle could be sourced from outside the 12 countries participating in the TPP and still be sold in the United States - the bloc's largest market - without tariffs.

(Reporting by Ana Isabel Martinex and Krista Hughes; Writing by Kevin Krolicki; Editing by Ken Wills)