Why is the Alternator gauge pegged out when the.....

So here is the long version, and this is really beginning to bug me pretty bad.
When starting my 03 Yukon, the alternator gauge is pegged all the way to the right.  The oil
pressure gauge is higher than normal and the speedometer gauge will be off higher or lower
and sometimes not work at all.  The Temperature gauge on the mirror works fine when all of
this is going on.  If the temp. gauge does not work, all the other problems are non existent.
And I always get a whistling noise from the radio speakers.  The radio is not the stock Bose
that should be in there.  I have had this checked out by a couple of different dealers, and they
always tell me they can't find anything wrong.  This tells me it might be quite complex.  Either
way, I want it fixed.  My 2 Cents:  I think the whole thing is tied together.  The stereo that is in
the truck now is aftermarket and was just rigged in to make it function.  At worst the
Temperature gauge in the mirror is defective and should be replaced.