My first CJ had no heat and only blew cold air and that was nearly 30+ years ago. Well My new JK defroster works nearly as good only melting the frost on the center of the windshield. Is there any mods or anything that I can do to get the defroster to melt the frost on the whole windshield . Other than an ice scraper.

 

Sounds like something is amiss. Possibly something blocking the duct. It is very easy for object to get between the front windshield and the dash.
Mine cleared my full front widow from last weeks ice storm. 1/4-1/2 inch of ice. Frost has never been an issue. And yes, a candle provides more heat than either of my old CJ's. Not to mention that wonderful weather tight soft top

 

So where should I start looking for this obstruction on the iside of the cab or under the hood?

 

My 08 Jk Rubi unlimited also only clears the middle of the windshield, driving in freezing fog yesterday the left part of the windshield from the top corner to about 12" in at the bottom iced up.

 

So far, mine has worked well at clearing the windshield. The replacement of your stock thermostat with one that opens at a higher temperature might put more heat into the interior heater core, and less into the radiator. This would put hotter air onto your windshield.

The other place to check is to see that you are getting good airflow to the windshield -- check that something has not fallen down the duct.

SV

 

Haven't had any trouble with ice yet, but I think the stock windshield defroster is inadequate at keeping the inside fog free in cold/rainy conditions.

 

crack one of your windows open an inch to keep them from fogging up in the rain....

 

You must be right on that because I definitely can't feel the compressor kick in like I did on the TJ, you knew that defroster was on!

 

That trick I am aware of, but because fogging is only an issue when it's cold and really rainy, it's not really a viable solution. Besides, this is a posh, refined, and comfortable JK, I can't have it feeling drafty and leaky like a "real" Jeep.

 

This is great news... It has always driven me nutts when the AC Comp runs when in def. It isn't needed at times, and other times it is; this gives me the choice!