Where is the summer fan switch located on a carrier weathermaker 8000?

You do not have a summer switch on this furnace, the only switch that you might have is the switch on the thermostat if it is a heating and cooling thermostat, if it is you have a switch that says "Fan Auto- On" In this case you would turn the switch to "On".
If I might ask why are you wanting a summer switch?

You do not have a summer switch on this furnace, the only switch that you might have is the switch on the thermostat if it is a heating and cooling thermostat, if it is you have a switch that says "Fan Auto- On" In this case you would turn the switch to "On".
If I might ask why are you wanting a summer switch?Isn't that obvious? You can switch the season back to Summer. If my system had one, I'd switch it too! Tired of this cold weather...

Years ago there was a white push pull button located on the fan and limit control located on the interior wall of the furnace. Most modern units stopped using this type of control and have gone to the circuit board and timmers to activate the fan.