Instead of installing a realy try this (it work well for me)
1. Connect your aftermarket fog lights to the stock wiring behind the front bumper.
2. Wire you aftermarket switch is series with the hot lead of the relay plug and the wire of the plug that goes to the fog lights.
Again, the plug for the realy is behind the driver's side kick panel.
Also, the techincal section of this website contains the complete wiring diagram for the celica. That diagram will tell you exactly what each wire is for the fog light harness as well as what color each wire should be.