this is a typical weld seal leak result. eg Part of sealing on a welded joint failing. under pressure of water allowing ingress. these can be a murder trying to pinpoint. ( I once traded a car in as we were never able to solve water in foot-well behind driver on a minder)
we have to look at every place eg water runs down as well as in a failed joint seal. check window seal on rear window. try looking in boot and around inner weld joints near foot well . behind wheel etc.
lift carpet. as far as possible and dry with hairdryer. pop along to washing area where high water pressure etc is applied or use a jet washer. sit in car and have water applied all over especially that area. if we can see ingress of water we will have idea where leak is and take remedial action. good luck. i wasted 6 weeks trying to find mine without luck.