Stock speakers on 1998-1999 RS's are 5.25 up front and 6.5 back.
Stock speakers on 2000-2001 RS's are 6.5" front and back.
To fit 6x9's in the back, you either have to 1) create a custom ring to raise the backet up off the floor of the deck so the speaker basket will clear the hole for the stock 6.5" speaker.
You will not be able to use the fibreboard rear deck cover this way.
2) cut into the sheet metal and fit it flush.
I pulled the 6.5"s right off my rear deck altogether.