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 panel.
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 involved.
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.
Rod