Every winter the drain of my Kenmore freezer clogs with ice every other day. (Every summer it remains clear.)

So I do what the repair guy did when it first happened: I boil a pot of water and melt the clog. Every other day.

How can I keep it clear during the winter? (Plus I'm curious, is this caused by room humidity or temperature differences?)

Manufactured May 1999

Without knowing what the ambient temperatures and humidity is where you live cannot even guess. If the unit is out in the garage, this can cause seasonal problems

I did go to the Sears parts website and did see that there is a screen on the drain, I would remove it as it could be impeding your drain flow.

Check that your door is sealing correctly.

I could not find a wiring diagram for your unit but can tell you that the unit has a 12 hour defrost timer unfortunately I do not know if it is continuous ( a defrost cycle every 12 hours) or cumulative (a defrost cycle every 12 hours of compressor run time)

Since you have been pouring hot water down the drain I am assuming that you have checked the drain tube for blockage and also that there is not dust impeding drainage flow at the drip tray.

Make sure the drain path to the drain tube is nice and clean,
Sometimes a coating of wax that has been buffed can speed up drainage flow.

There is a jiggery pokery fix for this that works for many people.
Take a 12 gauge bare copper wire and wrap it once or twice around the defrost heater. The other end is routed down the drain a couple inches.
Then every defrost cycle the wire gets warm and also defrosts the drain.
After installing this, I would then force a defrost cycle and let is run it's full time (usually half an hour or so) just to be sure that the wire just gets warm as you do not want to melt your freezer bottom. Perhaps instead of two wraps one wrap will do the job.
I would not do this if the drain is exposed due to safety concerns but if it is protected by a cover it should be OK.

I removed the cover over the evaporator, but I can't figure out where the defrost heater is. (But I think I may have found the timer's knob on the timer.)

So I took a couple photos, and scanned a couple parts diagrams:

http://coyote3.net/evaporator.jpg combines two photos (so I could show the whole evaporator on either side of the panel in the way; I can't see a way to remove the evaporator to see behind it; sorry you can't see the drain hole at this angle--it's in the middle below the evaporator).

http://coyote3.net/timer.jpg shows the back rear of the freezer. I put a red "=" sign on what I think is the timer; just to the left of the "=" sign is a black plastic dial that I guess is what I turn to advance to the next defrost cycle. (not exactly sure how to turn it, since nothing I have fits it, but I figure I'll push against one of the edges on it with a screwdriver.)

http://coyote3.net/diagrams.gif shows these areas and identifies the timer and defrost heater (though I remain clueless about where the defrost heater is in the photo!).

(I also uploaded these pictures in a zip file at https://www.yousendit.com/download/eUREbUphbEozeUpjR0E9PQ 3MB; I first made versions that met the filesize requirements for Reply attachments, but it didn't work for me, sorry!)

please help me figure out where the defroster is!

thank you!
coyote

Thank you denman!

That "black device (wire)" is coated (with 'plastic'), so now I don’t know how to make your exciting advice to wrap bare copper wire around it work (because I imagine that I need to make the bare copper wire contact bare metal).Yes! That worked great (and was fun), thank you again denman!

There doe not seem to be a way to delete a post.

Sort of got confused by your last post.

I am assuming that you have been able to force a defrost but am not sure if you were able to install a drain heater.

The heating coil looks like it is held onto the evaporator coils with a couple of wire ties. You could undo these to make access to the heater easier. Then wrap the copper wire around the heater a couple times.
Just be careful that you wrap it just snug not tight. You do not want to compress the heater.