Read carefully. The entire unit comes out of the dash. There are spring clips on each side to hold it in place, so that coming out is rather difficult.
The dial knob can be pulled off and you can spray cleaner down the shaft.

Remember, the instant mpg setting does not work when the car is not moving. Reseting the trip miles should be done when the car is stationary, like at a gas station.
For really good mpg readings, just do it everytime you tank up. Divide the miles traveled by the gallons used. The computer uses averages of fuel rate and may not be that accurate.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)