I do not imagine that it could be all that difficult as my 2007 came with Sirius installed. I think that you just need to swap out the receiver module that lives behind the radio console.
he part number for a sirius tuner on a 2007 Toyota Camry
WITH OUT NAV. CAM/SOL SATELLITE RADIO(SIRIUS) Receiver Kit
Part Number: PTS31-00052
Fit kit (antenna and cables) for camry without nav
SATELLITE RADIO (SIRIUS) Fit Kit
Part Number: PTS31-00051
..... or if you have the navigation
WITH NAV SIRIUS RECEIVER KIT
Part Number: PTS31-00052
Fit kit for nav
SATELLITE RADIO (SIRIUS) FIT KIT
Part Number: PTS31-33070
http://www.buy.com/prod/sirius-scc1-...204832843.html = $46