on the 89 dynasty, it is located right next to the thermostat
When the blower motor stays on it means there is a short to power, or the relay is sticking. 99.9% of the time the relay sticks. I belive on the 2004 silverados the blower motor relay is internal to the blower motor resistor. Try to find an auxillary blower motor relay and if there isn't one then replace the blower motor resistor.