rear wheel bearings

replace the same one twice in less that 3 months what is causiing them to fail . it's the same one on the passnger's right rear. don't kno whats causing it i think it my be cause i 'm usin those value craft parts from autozone th're $ 19.99 a part should i use the ones that cost more.
ALSO IT'S A 1994 LX 3.8 NON SUPER CHARGED.

I used to buy CR bearings from Pep Boys it was a decent bearing. I've also used National and Timken.
Make sure the bearing is lubricated before you install it. Check your rear end level and make sure its full. Use a new seal when you install the bearing. If you have found metal particles flush the rear end.

Direct from Ford part for less the $13!! http://www.trademotion.com/partlocator/index.cfm?action=getLocator&siteid=213787&chapter=DP2FD0&appSectionid=23&groupid=10047&subgroupid=60086&make=10&model=Thunderbird&year=1994&catalogid=1