T660 heater fan stops for hours and windows fog up.

My old T600 was doing the same thing. Almost bet your problem is the same as mine. Check your blower relay, Its heating up. Then kicking out, thus the working some.
Mine got where it was kicking out every minute or so. Replaced the breaker. It now runs perfect.

Just a heads up do not touch that breaker when it just kicked out. I branded that finger for a couple of weeks. I used a cheap temp gun from Harbor Freight to find it. Sure saves skin when its hurning hot.

 

Took the words right out my mouth.

 

Got mine off Ebay. Guy had a bunch of new ones listed. I think with shipping it was less then $5.

 

They asked me $12. with a Fleet Account.

 

Where was the breaker located?

 

I'm currently having the same problem, usually goes out after I shut my truck off, if I turn it back on nothing for hours. Leave the truck shut off come back later... Works. I can find the sleeper relay but that it. I've checked my power to the plugs and everything. When it works it works on all speeds. Only thing I haven't done yet is open up the heater box under the hood.