hi chewkies...
you should provide more info about the problem that you have facing.... like does the carpet get wet only when you drive in the rain.... or does it get wet just by parking in the rain or when you go for a car wash...
if it gets wet in a carwash or stationary in the rain, then there is a leak somewhere in the rubber.... could be the from anywhere.....
if water only comes in when you drive, then there is a hole at the undercarriage.... there is a cap at your undercarriage... check if the cap is still in tact.... if it is missing, get a replacement.... if it is still there, you may want to check if it is sealed properly.... the size of cap is a little bigger than a 50 cent.... try it out and see if that is the source....
cheers,
andrew_e30