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Mike the doubler has an adjustment knob on it that allows you to set the max output. You have to pull the knob up or out and then turn it to adjust the max pressure then push it down to lock it in place.
Newell started installing them on the passenger side of the engine firewall, usually above the fuel filter like Chester said. Not sure when but my '98 didn't have one. It should look similar to the picture in the link Richard posted. You would hook your air hose to it to get the higher pressure air.