What is the burden of proving consent in a search warrant application?

The People had the burden of proving that defendant's manifestation of consent was the product of his free will and was not a mere submission to an express or implied assertion of authority. (People v. Johnson (1968) 68 Cal.2d 629, 632, 68 Cal.Rptr. 441, 440 P.2d 921.) We believe that the officer's testimony on the issue of consent met that burden [114 Cal.App.3d 72] and that substantial evidence supports the trial court's conclusion that the defendant voluntarily consented to the search of his vehicle.