Did the instructions tell you what the diameter of the U shaped wire
needed to be?
There is a specific diameter and if you are using a wire that is
bigger, it will not go all of the way in and if you are using a wire
that is smaller than called for then it will go all of the way in but
will not depress the pins that keep the black, hardened steel cone
from coming off. Also the key must be turned to the first position
otherwise anyone could steal this vehicle using only the wire.
There are other problems with many Mercedes ignition housings that
would make one think that the keyed cylinder is faulty or worn when it
is not. Specifically the column locking cam that sits behind the
ignition lock wears out making it really difficult to get your key to
turn to on. Once you get the key to turn, everything works beautiful
until you pull the key out and hear the locking pin clicking into lock
position then you have to start all over again. When this condition
happens, you can sometimes temporarily free it up using a deadblow
hammer on the locking bolt shaft after removing the kick/service
In either case, lubricating your lock(s) will not hurt at all, fact is
if the tumblers are the problem then using a good lubricant, either
silicone or a teflon based like TriFlo, should help immensly, however
lube doesn't really seem to help if the problem is a worn lock bolt
cam. Remember WD40 can clean a lock out but as far as supplying
lubricant, forget it.
To solve the lock bolt problem if you have one is quite a bit more
Another possibility is the key itself, either order a new key from
Mercedes, they will cut it from the VIN to original factory specs or
go to a locksmith that can do the same, the key can even be cut a few
thousands higher to compensate for tumbler wear. Check prices and
decide. If the key is worn, the lock can act as you say it does and a
new key will often work like a miracle.