Clunkfish

It looks like it might be a piece of aftermarket trim applied on top of the original? I had that in my 1998 LS400: it's a real pain to get it off if that's what it is :-(

Also didn't they have cabin filters on those years? Make sure its clean too.

I second that. When I replaced my cabin filter the air flow increased very noticeably. You can find it behind the glove compartment: there's some very helpful info here http://us.lexusownersclub.com/forums/topic/34808-how-to-replace-cabin-air-filter/

It needs more than a shake - best to blast it (carefully!) with an air line to get all the dust out (there will be LOADS). As I say, if removing the filter makes a noticeable difference to air flow and temperature (it did in my Mk IV), then replacing it is indicated.

I would also make the sure the cabin air filter is not clogged. It can accumulate a lot of fine dust as well as the obvious leaves and stuff. Try running the A/C with the cabin air filter removed - does that make a difference? If so, replacement filters are not too expensive.

Just to say, water is heavier than fuel so the fuel will float on the water, not the other way round ;-)

Try http://lexus-europe.epc-data.com/ls400/ucf20l/5090/electric/8716/87240/

Prices can be given in Euros, so presumably they'll ship to Spain :-)