I'm trying to diagnose and fix a problem with the heater on my wife's '97 Fiesta. I know there is a well-documented issue with the heater control valve (HCV), which electronically controls how much coolant flows through the heater matrix.

The HCV has been changed approx. 6 months ago, but the problem has returned. I have found a guide on the web on how to test the control panel for the HCV, which sends out a pulsed signal to the HCV. It mentions using a Test Lamp to check for a pulsed signal, "To check this you need to connect a test light to the live side of the battery and the other side to the black / green wire, then turn you temperature half way between min / max you should see the light be either on all the time or it will pulse on and off every minute or so"

What I want to know is, can I use a multimeter instead, as I don't have a test lamp? If so, what would be the best way to wire it up?

That would depend on how fast the multimeter reacts and how fast the pulse is.
Sometimes to see the pulse its best to use the basics and the bulb trick would be a better solution.