Or, if you want to do it yourself ...

1) Locate the speedometer cable coupling box.  On my '84 it is
   underneath, just behind the spare tire.  If I remember correctly,
   it is about 3x3x1 inches with two speedo cable couplings and some
   wires attached to it.

2) Look for either a small hole, or a pin protruding through a small
   hole.

3) Push the pin back into the hole, or further into the hole if it
   doesn't protrude.  You will have to use some sort of tool to do
   this since the pin will lock in the reset position when it is
   somewhat recessed.

Remember that the idea here is to replace the sensor and then reset the 
reminder circuit.

Mike