Sounds like a bad ground to me.

 Guage fluctuation can be a sign of a regulator problem, or an old
alternator which might need attention/replacement.  It could also be a sign
of nothing more than a bad ground....uhmmm...somewhere on the car...(Yeah, I
know...REAL HELPFUL there, eh?  ;)

 The light symptoms are quite similar...sometimes you'll have nothing more
than a bad bulb that'll throw things totally out of whack (ie: one bad turn
signal bulb bungs up the flasher unit), but it might als be related to the
above problem, and once again it leads to a possibility of a bad
ground...somewhere...

 To FIND the problem, plan to spend an afternoon tracing wires...front to
back, and back to front again...until you find it, as it can literally be
ANYWHERE on the car!

    First on the checklist: BATTERY TERMINALS!  You'd be surprised how
often that goes overlooked!  Even if it's not the cause of your current
problem, acid DOES tend to seep into the connections and creates some rather
nifty deposits that, over time, can seriously impede charging, drawing from
the battery (ie: starting) or (this is where it gets obvious) the main
ground from the battery.  Just because you've got a "permanently sealed,
maintenance-free battery" does NOT mean that you're immune at the terminals!
Just had to spend a good half-hour working mine over here...they were ripe!
;)

 I'd also suggest checking the rear lighting harnesses next, since that's
the next most obvious starting point, as it seems to affect your lighting.

 Get yourself a GOOD shop manual with wiring diagrams, as you'll find that
those really come in handy when you're trying to figure out where all the
*@# those *@#in' wires are coming from/going to...the more complex these
damned electrical systems get, the stranger connections you see...

 I had a dead dash once, and after days of trying to trace the error, it
turned out to be a bad wire on a sending switch...on the transmission!

 The moral of the story?  Check the most likely spots first...but don't
expect to find it either "quickly" or "on purpose" because it could
literally be ANYWHERE on the car.  You might also want  try keeping a
written log of the wires you have checked, as o may be able to save some
time in duplicating "apparrently different" wires which are actually running
the same connections you've already tested...

 DISCLAIMER:  "Or I could be wrong..."  ;)