You should pick yourself up a cheap multimeter...it'll help loads in accurately diagnosing your problem. You need to assess:
1) battery voltage -engine off
2) battery voltage -engine on @ idle, after a quick rev)
3) battery voltage -under load (headlamps,blower)
4) battery voltage -at 2000rpm or so under no load
5) battery voltage -at 2000rpm or so under load (headlamps,blower)
A cheap meter like this one will help
http://www.oreillyauto.com/site/c/detail/WMR0/W2974.oap?keyword=multimeters+%26+analyzers
It very well might be your regulator, I just replaced mine in my SD 1/2010..and its not output much voltage. The alternator itself i believe is OK. I'm going to order a replacement and we'll see.
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