I accidentally broke this piece of plastic under the wiper. Although I don't think it is a critical part, does anyone have an idea what is the function of this little part? Thanks!

 

I'm not 100% sure which "little part" you're referring to, but, if you're talking about the piece where the red arrow is pointing in the photo below, I suspect it's a windshield wiper air deflector. It is used to help keep the wiper pressed against the windshield. However, I don't believe this is an OEM part, at least not the US model LS430.

 

It's the black square block thing in the picture, that piece of plastic lying next to it is the bit that I broke

 

I don't recognize any of the arms or the "block" on my 2004 LS430. Do you have right seat drive in your car?

 

It is indeed a right hand drive. Maybe it's only on the rhd versions but I checked the parts diagram of the toyota celsior and still couldn't find any information.