Flood the lock with WD40 or other penetrating oil by putting the straw into the keyhole as far as it will go and spray for a few seconds.
Repeat this every 10 minutes, occasionally working the key from side to side without undue force.
When flooding the lock, have the boot lid in the open position so as to allow the oil to drain down into all of the lock mechanism.
Its unlikely the lock has been broken, probably seized due to lack of lubrication and usage.
See also this W202 boot lock, different design from yours but will be similar to what is inside your lock.
WD40 is not a lubricant but it is very good penetrating oil, that is what you need now.
I don’t think graphite powder is going to get much further past the lock barrel with all its levers and through to where it is needed, at the mechanism behind the barrel as described in Picture P, however a liquid oil will, provided the boot lid is in the open position so as it can drain downwards.
I have heard that WD40 isn’t good for car locks too but I don’t believe it, I think somebody was spraying a car lock with it, then took the lock apart, found all the corrosion/oxidization similar to the pictures and blamed it all on the WD40.
The brass levers will oxidize/rust so will the main part of the lock, which is an aluminum alloy, this will oxidize/rust too. The best prevention is a silicone spray that won’t dry out, used a couple of times a year and the usage of the key in the lock occasionally will keep every thing in order.
Give it another good flooding with the boot lid open, the WD40 will dry out in about a week and will no longer act as a lubricant.
Get a spray can of silicon lubricant something like Holts Rubber & Nylon Lubricant and give it a dose of that, then 2 or 3 times a year, use the key occasionally too.
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