Do you have a stalk switch or a dash switch? As far as I know. the factory wiring on the '99 SR5 is to the stalk, you just need a different stalk to make it work.
To use a dash panel switch, I assume you would have to patch into the stalk wiring. I wired mine from scratch and added a dash switch.
I read some where that you can buy the oem switch for $10-15 at the dealership