Im looking to get a 98-00 Lexus GS, and want to add the aftermarket Pioneer Navigation system. I went to my local stereo shop and they said that my lexus requires a line output converter for $100 in order for my system to work correctly. They told me that the line output converter is required to make the stock amp work or something like that. I think the store is trying to rip me off, I thought all i really needed was a wiring harness and a dask kit. Can anyone help telling me what I need to buy to make the system work correctly?

Thanks for your help