your voltage regulator is inside the alternator but replaceable from 
the exterior.  a new one should be mounted on a brush holder and is a 
straight bolt in item.  $15.

a rebuilt alt. for $89 sounds like the begining of a bad, long 
relationship to me.  there are some truly horrible rebuilt items out 
there and you are better off having your alt rebuilt.  cost about 
$50, including a new brush/regulator!

now *is* the time to have the work done as preventative maintenance.

-dan