Hyundai Repair: Question, hyundai elantra, digital voltmeter

Question
I have a Hyundai Elantra 1995, and when I press on my rear defrost button it doesn't click (stay on). It's a new button that I put in and it still does the same. I would like to know if there's a fuse or a rear defog relay somewhere. If not, can you help me figure out where the problem is coming from? Thank you in advance.

Answer
This will probably take a few questions to sort out what sort of problem you're having with the system.  

Does the indicator come on when the button is depressed and go back out when released?  If so, it's likely that the wrong connector is plugged into the defogger switch.

If the indicator lamp doesn't come on at all, then the ETACM (electronic time and alarm control unit) isn't attempting to turn the defogger on.  In that case, you'll want to check the defogger fuse (typically about a 30A fuse in the interior fuse box labeled something similar to "rear heated").  If that's okay, start your engine and check the voltage across the battery terminals with a digital voltmeter.  It should be at least 13.5V while the engine is running.  If not, the ETACM won't try to turn on the defogger because it believes the engine is not running.  In that case, you'll need to diagnose why your alternator is not working.