My tacho doesn't work. Hasn't in the time I've had the car. I've had the back off it, played with the dip switches there and still nothing.

It's a Caerbont branded tacho (who make Smiths gauges) and I'm running a megasquirt.

Now, pretend I know nothing about how tachos work, should I be looking for a feed directly out of the megasquirt that tells the tacho what's going on, like a voltage to indicate the rpm? Or does that come from some other source?

Their are a number of tachometer feeds available but I'm guessing you don't have an EDIS so you can feed from the Megajolt or the coil pack.
To feed from the coil pack you'll need the diode pack as shown on the Megajolt website
Works with Caerbont as I ran mine from the coil pack with the diodes
You should be able to feed from the Megasquirt but need to use the correct wires.