[john70t]

(keep it out of gear obviously..)

You can jump past the key/relay/solenoid with a jumper cable direct to the starter solenoid positive terminal.
That should engage the bendix and spin the engine.
If there is proper spark and fuel it should start.

Are the readings Key-on, or in Start position for the ignition switch?
Control side, or Load side of the relay?

0 volts would indicate electricity is passing through the relay to ground somewhere when the wire is connected. The load is elsewhere. The flow undetermined.

As mentioned it could be not enough amps to activate the relay or solenoid due to a poor ground or positive side connections.
You can jumper another new ground from engine/chassis/battery neg easily.
A dozen new grounds won't hurt anything.