I too got the nondescript 1320 error code on my 2000 GLE (120K miles)
and opted to replace all the coils rather than fool around trying to
determine which one(s) were acting up - especially when there is such
a history of problems associated with the original coils as any search
of this newsgroup will reveal.

A number of months prior to getting the code I noticed that my gas
mileage seemed to be getting worse and the engine would ping when put
under strain such as rapid acceleration or going up a steep hill --
even though I always have used premium gas.  These problems seemed to
be getting progressively worse over time.  About the time the code
first appeared the engine would intermittently idle roughly.

I bought the coils over the web for less than $500 and installed them
myself in about an hour - a very easy job.  Nary a ping have I heard
since and my mileage seems to have improved to the tune of > 10%.

If you haven't yet replaced your original spark plugs at 94K miles
you might as well do that too when replacing the coils.  As for the
"Check and replace wiring harness" advise you were given I suggest
saving your time and money by focusing on replacing the coil(s), which
is the far more likely cause of your problem.

Good Luck,

Feynmanfan