Hi Bill
Got to say not something I have come across. I take it the weedkiller was in liquid form.
I would reckon thorough flushing with a standard detergent would be suitable treatment.
One word of caution if it has melted rubber seals, it may also affect the carpet base which in my Landrover has some kind of foam attached to the back. Did the carpet appear dimpled or ruffled at all?