This is probably pretty obvious for quite a few of you, but it took me a couple different sources to get this done so I thought I would throw something together for spoon feeding. Joe Perez had a picture up at one point but imageshack lost it.
This works for 1.6 Miatas without Cruise or Auto trans, any other Miata should already have the VSS signal passed to the ECU. For MS3x (and some Reverant ECUs) no extra circuit is needed, for earlier Megasquirts a pull-up resistor is required. This just covers the wiring from the gauge cluster to the ECU. Read here
for pull-up info.
Remove gauge hood (and then try and plastic weld the tabs your broke off)
Remove the gauges from the car
Find the screw terminal labeled RSW
Add wire to the terminal
Run the wire to where your megasquirt is
Leave some slack for when you want to pull the dash again:
Put everything back together.
Wire it into the Datalog In (or tableswitch, cam in, etc)
These settings worked well for me: