mattdel

The #1 reason for repeated heater core failure is electrolysis in your cooling system. This happens from having bad grounds, not enough grounds, etc...
To test, take a standard volt meter and insert the + probe into your coolant, with the - probe on the battery. If you get a reading higher than 0.2V, you're going to be doing heater cores yearly.
It has nothing to do with who you get the core from, what brand name it is, etc..

If electrolysis if found to be a problem, correct any ground wire/strap problems, flush the cooling system(don't just drain the rad), replace the core, and add a ground strap to the core itself with a small weld.
Problem solved for years to come.

sscobra

Yeap, that is the first correct answer. Also, another thing that will help is adding zinc anode to the radiator. The electrlysis will attack the anode and not the rest of teh cooling system. These can last 1-2 years and can be changed when doing coolant changes yearly. Also, never use tap water to fill the cooling system. The minerals in tap water will destroy the heater core along with any aluminum parts.

88BlueGT

I did the same thing almost, I bought a heater core from autozone, put it in and than took it out before even running the car and replaced it with a ford core after comming to my senses.

kaggriff

another thing that will help is to drop the pressure cap from about 16psi to a 11 or 12. plenty of pressure to keep the boiling point high enough. with the high poundage caps it puts a lot of stress on these older parts. but just my 2 cents on the subject.