There is normally a connector for the white wire, but we are only concerned with the yellow wire at this point. If you are getting 28VAC, then the lighting coil is working. The only question is how much current it can provide. This is not important at the moment.
I believe the regulator is a one wire device, so yes the regulator base must be grounded. If you have no voltage at the regulator input wire, it is most likely that you have a bad connection somewhere.
It's time to cut the harness open to trace all the yellow wires. Follow the yellow wire that connects to your stator yellow wire. You should be able to measure your 28VAC as you move away from the stator (bike running).
The regulator is supposed to clip excess voltage (anything over 14VAC). The only way you'd get nothing is if the regulator itself is short circuited to ground internally or there is no power getting to it. Now we are back to bad or open connections somewhere.
If the regulator wire is disconnected, do you get 28VAC coming in on the mating wire?