Suspect Firewall connection block.

A. There is no fuse on the ignition circuit.
B. Amount of voltage drop depends on both resistance and flow.
Resistance x flow = Voltage drop
You can't check for resistance without knowing how much current is being pulled and the route the current is taking.
Also measuring to ground can be misleading. Its good to know the voltage available at the power source but measure the votlage differnece directly when possible.