'98 Toyota Avalon heater only operating on low

'98 Toyota Avalon heater switch is only operating on low. Anyone know what I need to look for and where it’s located to repair this issue?

That sounds like the resistors. The fan speed is controlled by using resistors to slow the fan down. In time they often burn out, one at a time. After the first one goes the others start going soon because you are using them more.

The specific fix depends on the your car.  Sorry I don't know it well.