Assuming that 79's remained the same as earlier years, you should have 12v coming into the kill switch on the "white" wire. Should be the same exiting the switch on the "white/yellow" wire headed to the ignition coils. Please note that depending on your switch type, the white wire can be bridged between a couple of the switches, so you will need to check it at all points prior to the kill switch.
I would start with a simple voltage test first for verification. Just in case there is a parasitic draw at a coil, etc, disconnect the "white/yellow" spade from the coil terminal. Make sure you have battery voltage coming from the "white" wire into the kill switch (should be fed by the ignition lock circuit). If you do, great, if not, trace the white wire back towards the lock cylinder, or skip a step and check it straight at the lock cylinder.
Provided you are getting 12volts at the white wire at the switch, check for 12v coming out of the switch on the white/yellow wire. I would cycle the button while you are checking to see if voltage changes. Should go from 12v to 0v and back repeatedly. You can check it at the coil connection spade, or closer to the switch itself. Would be worthwhile to check both.
I expect you will find a large variance in the voltage readings between "in" and "out" sides. If not, next step would be to check resistance. Reistance should be nominal (around 2.5milli-ohms per foot of length). Most likely culprits are the switches, connectors, or a break in the wire. Multistranded wire doesn't break really easily, so expect the first 2. You can check each component accordingly in isolation if need be (white/yellow wire, white wire, switch itself, etc).
If everything checks solid on voltage and resistance, check the coil grounds for issues. After that, assuming you still have the Lucas ignition, check the EI Box following the test proceedures outlined in your manual. In a pinch, you can find the test proceedures here (last couple pages):
Hope this helps!
P.S. Almost forgot, I would run your meter ground back to the battery and make sure it is a solid connection for the ground, otherwise everything else goes out the window. You can use a frame ground, but not always the best choice...