I have a 1991 Toyota Celsior UCF11 in the UK.
I have the familar problem of the aircon/venilation fan is blowing
properly but little to nothing comes out of the vent.

I was told to replace or remove the aircon filter.
But the LS400 wasn't fitted with one until 1993, and while I have been
given instructions on how to replace the jap spec celior ones, I can't
find it in my car.

Instructions I have seen suggest, on early models, the filter is behind
the knee bolster under the glove box, extracted from below by removing
two wingnuts, while later models are accessed from the glove box through
small hatch. I can't find either ways to access the filter. There must
be one as it is blocking the flow of air from the vents almost totally,
despite the fan speed/noise increasing.