E36 waterlogged!

Please help!!! I have a rather waterlogged 352 TDS SE. It seems to be RH i.e drivers-side only (UK) and has been a puzzle for 18 months now. Sunroof drains seem to be OK but found a floor plug dislodged a year ago (could have driven water up into car in during wet driving) but it still is getting in and can leave half inch of water in there. This is not damp, this is WET! Can anyone suggest a list of things/places to check?

TIA
:)

Tim