Hi,

floor pan. I have a 405 - totally different car I agree, but under the
seat is a big round plug about 125mm diameter from memory which gives
access to the fuel gauge sender and the top of the tank. I would imagine
if this did not seal properly then when driving in the rain, water would
be driven in through this plug and hence into the seat area.
Brilliant. I haven't lifted the rear seats for ages (I have no big needs as
a cargo), but it seems to me there are 2 of these big plugs.

Regards,
--
G.T