I need detailed instructions on how to locate the oil pressure sending
unit on my 1988 Mercedes Benz 420 SEL, and, once located, how to
disconnect it.  Pictures or really specific instructions please, as
I'm basically totally unfamiliar with cars.  My goal is to have the
gauge always read 'full pressure.'  I've had the sending unit
replaced, and the gauge continues to bounce between no pressure and
full pressure pretty much randomly.  It's my understanding that
removing the connection or the ground or both will peg the gauge at
'full pressure,' which is what I want.  (I know the pressure is good,
had it tested, and there's the oil light for back-up.) Thanks.

I don't know the exact location of the sending unit on this particular
vehicle, but would highly recommend against what you are trying to do.
Yes, the gauge may be fluctuating quite a bit, but it IS giving you
some indication that things are working. Imagine this scenario:

You're driving and hit some junk on the road. For argument sake, lets
say you hit a piece of rebar (Steel rods used in construction). This
errant piece of metal happens to pierce your oil pan. Just enough to
cause an oil leak. With your gauge "pegged" you'll drive along, fat,
dumb, and happy until the engine begins to overheat (rather quickly
with oil starvation), and causing thousands of dollars worth of
damage.

My bet is that the problem is more than likely a bad ground for the
sensor. I would seriously reconsider hot-wiring the gage, and would
try to find out what the cause of the problem actually is.

Just my .02

Would the oil light be connected to the guage, just coming on as a danger signal?