I have a Kenwood sirius setup with Kenwood cd headunit. First, there was no auxilury inputs on my 2000 stock unit so I assume yours is same, only way would be to disable tape deck, but would require some prior knowledge.
I installed the whole setup my self (took many hours to make it look nice)
Is your celica a gt or gts, if its a gt does it have the premium sound package with a sub above passanger footspace(should be able to see).If its a gt with non premium sound you should be able to get a harness adaptor for a new headunit.I have a gts so I had to cut all the wires before they enter the factory amp, and re splice them to new harness (found guide on this site)
Also, if you haven't already made a choice, I would definately recommend sirius over xm (sirius has no commercials). Plus sirius has more future potential.