Alt volts

Your relay drawing is almost right. The blue wire (ign1) goes to coil+ (86?), but coil- (85?) must be grounded (you don't). Check the back of the Radio Shack package since the coil polarity matters if there is a diode across the coil. I run +12 V supply to the sideways pin (30). The output to your voltage regulator is pin 87. Some relays have a pin 87a with is "normally closed" if you need something to open when the coil energizes.

The reason you need a relay is because if you hooked Vreg sense directly to the battery, it would draw some current with the ignition off and could run down your battery. Of course, new cars have many loads on the battery when off, which is why airport parking lots regularly jump people who left their new car >2 weeks.