Hmmmmm "the plugs from hell".

These plugs are found in a metal bracket underneath the LHM tank (big black
plastic thing with pipes coming from the top and the little glass dome where
you check the Hydraulic Fluid [LHM] level).

They just the most aggravating things in the world - the entire engine
management wiring harness runs through them as well as lots of other
important things.

People have suggested in the past that it is spilled or leaked LHM that gums
them up but this is not actually the case.  The plugs are of such bad
quality that the contacts of the female side get pushed back inside the plug
casing and so they don't make contact anymore or worse still do this
intermittently!

To get to them you have to unclip the LHM tank (there is a kind of spring
clip mechanism at the front near the air cleaner) and gently manoeuvre it
out of the way and you will just be able to see the plugs underneath.

This is not just something I've encountered - every XM owner knows about
these two examples of French engineering and has questioned the parentage of
the designer or accountant who sanctioned their use on what is generally a
good car....

My advice to you (if you haven't done so already) is to make your way over
to the XM mailing list (http://autos.groups.yahoo.com/group/XM-L/) and have
a look through the archive - you will find many tales of woe resolved by
replacing these "less than satisfactory" items!

HTH

CAS