Ambac Financial Group’s regulator won support from Dunkin Brands, Sonic Corp. and Hertz Corp. as he seeks to overcome objections from some of the insurer’s clients to his plan to rehabilitate the bond guarantor. Executives of donut retailer Dunkin Brands, drive-in restaurateur Sonic and car-rental firm Hertz, which issued bonds insured by Ambac, filed affidavits in support of Wisconsin Insurance Commissioners Sean Dilweg’s motion in state court yesterday opposing the legal bids by two groups of bondholders.