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A 1990 to 1994 is under the glove compartment it is not a simple deal to get to it, the panels need to come off and the glove compartment needs to come out to do it.
I have posted a write-up, for the Gen I LS400, on how to access the ECU on the other big Lexus forum - it is the US Owners one - look for the "How-To" write-up under the "guides" by fsuguy