I just hooked up my battery after 8 years and a complete teardown restoration. In the process of troubleshooting a minor electrical issue, I noticed that with the ignition set to run, after a few minutes, one of the steering column harness connectors became warm to the touch. NOT hot, just warm. I assume it was the ignition harness, there were only about 5 wires in it, one red, and they were all thicker gauge.

Is this normal? I know it shouldn't be left on "run" for a long time without actually running, had to leave it on briefly for troubleshooting. Thank you.

No ..it's not normal. High resistance causes it and can leed to a meltdown.
First, I would check all electrical connections and make sure they are "set". If they are I would check each pin "to ground" with a meter... shouldn't be any reading. If that checks ok, then pull the harness and look for a burnt wire or overheated wire sheathing.

If you know what wire it is then disregard checking the wires. But check it to ground, etc,etc. etc..

ps..Another thing that should be done on an old car is pull the bulkhead connectors and look for corrosion. If they are original, they'll have (green)crud in it. I use Pepsi to clean the copper terminals. Pour some in a cup and dip the harness connector in it for about 10 minutes. Rinse off with water,dry and use a spray like WD40 or even white grease.

So by checking for continuity to ground, you're looking for a short? In some cases, wouldn't a ground be normal? And a ground would be a short, which would be low resistance, but would trip the fuse, right?

Sometimes a meter will show grounded. Like the starter circuit when the key is turned to the start position. "High" resistance is bad and "low" is good or better. Not all wires are fused. Some are protected by the fusable link to prevent a meltdown in case of a short and by the time you know, it's too late. A grounded wire and resistance are two different things. Resistance is the how much effort it takes for electrical current to flow.

If it's only with the key turned to "on", it's a matter of isolating it. Look at any circuit that is powered then..like the alternator/harness, heater switch, turn signal, etc...

Electrical resistance
Voltage can be thought of as the pressure pushing charges along a conductor, while the electrical resistance of a conductor is a measure of how difficult it is to push the charges along. Using the flow analogy, electrical resistance is similar to friction. For water flowing through a pipe, a long narrow pipe provides more resistance to the flow than does a short fat pipe. The same applies for flowing currents: long thin wires provide more resistance than do short thick wires.

The resistance (R) of a material depends on its length, cross-sectional area, and the resistivity (the Greek letter rho), a number that depends on the material.

The resistivity and conductivity are inversely related. Good conductors have low resistivity, while poor conductors (insulators) have resistivities that can be 20 orders of magnitude larger.

Resistance also depends on temperature, usually increasing as the temperature increases. For reasonably small changes in temperature, the change in resistivity, and therefore the change in resistance, is proportional to the temperature change.

At low temperatures some materials, known as superconductors, have no resistance at all. Resistance in wires produces a loss of energy (usually in the form of heat), so materials with no resistance produce no energy loss when currents pass through them.

Ohm's Law
In many materials, the voltage and resistance are connected by Ohm's Law:

Ohm's Law : V = IR