Probably overstepping, but maybe a description of what a relay does will help.
A relay is composed of a normally open switch and a solenoid that operates that switch.
The switch part of the relay is wired to your 12v source (in this case your battery) on one side, and the component you are powering ( in this case your hei dist.) on the other. The solenoid, which operates the switch is wired to the ign. terminal on your ignition switch. The ground wire on a relay is for the solenoid part of the relay. The basic idea here is that with a relay, you are indirectly powering your hei from your ignition switch. When 12v is applied to the solenoid it closes switch which allows the 12v from your battery to power the hei. That way, your hei gets 12v from a short run of wire from you battery which avoids voltage loss caused by the resistance that accompanies a long run of wire (I.e.: from the battery to your ignition switch and back). The solenoid part of the relay draws very little current so that voltage loss is not critical, hence the gauge of wire from the ign. terminal of your ignition switch does not have to be that heavy (14ga. would be sufficient. )
Hope this helps.
Apologies if you already know this stuff....
