I have had problems with my windscreen and front windows being fogged up all the time unless i have a window cracked open.
As well as this I have been using a small amount of coolant ( maybe a pint over 4weeks)
From this and the fact that I tasted the condensate on the inside of the windscreen and it was sweet , the same as the coolant, I deduced that my heater matrix must have a very slight leak.
After looking into the mighty job that replacing the matrix involves I decided at first to try some rad seal.
After some research I came across a product here in the UK called K Seal.
It cost about £15 and I poured it into the radiator once some coolant had been removed by blowing into the expansion tank (covering the overflow pipe)
After about 10mins the screen went clear and I now enjoy hot dry air in my XJ and clear windows. Just thought I would share this with everyone.

 

Good deal! Hope it holds for you.

 

Is that kind of a ceramic stuff? We used something like that on our big diesels. Worked well.

 

Yeah..its ceramic and copper according to the website.

http://www.k-seal.co.uk/

Not sure how long it will hold but i will let you know if it eventually fails.
I was going to write my Jeep off and get another as the time and hassle of tearing out the dash was too much. A bit extreme I know but in the UK you can pick up the very best XJ,s for under £2K.
Actually I reckon I would have fixed it, that's why I bought the Jeep, so I could do a few jobs myself.

 

Honestly, I never trust any additives to stop a coolant leak. They'll cause damage to your water pump, and they can clog smaller coolant passages.

Heater cores are a pain on just about any vehicle (just helped a friend do one on a '92 Camaro, took three hours), but it's worth it to not pour copper sludge into your cooling system.

 

fleetguard- a Cummins brand has come out with whats called super sealer which they have been running in their big diesels for 500000 miles to reseal a leaking egr cooler instead of having to pay 2500 dollars to replace it. they have not seen any issues out of it. if you can get a holod of it i would trust it for something like this. its really thin and does not destroy seals or block up passages. it bonds with cracks. it would work on a slightly leaking heatercore i would imagine.

 

I miss my '89 Ranger. It had a little access panel held with 2 screws just for the heater core. About a 20 minute job start to finish. Pull the dash to get at the blower or heater core... ugh! Same thing with my 02 Chevy Tk.