In Montrose Chemical Corp. v. Superior Court (1993) 6 Cal. 4th 287 [24 Cal. Rptr. 2d 467, 861 P.2d 1153], the court explained the parties' burdens of proof in a declaratory relief action regarding the duty to defend: "To prevail, the insured must prove the existence of a potential for coverage, while the insurer must establish the absence of any such potential. In other words, the insured need only show that the underlying claim may fall within policy coverage; the insurer must prove it cannot." (Id. at p. 300.) The Montrose test is applicable to an action between successive third party liability insurers. (Maryland Casualty Co. v. National American Ins. Co. (1996) 48 Cal. App. 4th 1822, 1831 [56 Cal. Rptr. 2d 498].)