Good advise from railman .
The lock is a rather cheap affair , and dis-assembly is what I'd do - but I was a locksmith when going through college .
Short of disassembly , you could use clean compressed air to help blow out the residues in the lock (prob. have to do it several/many times) & I'd suggest that you use a couple of drops of iso alcohol 90% in the lock when doing so .
Make sure you have some sort of absorbent rag for catching the 'mud' / alcohol etc. , if there's openings in the lock inside of your trunk .
After you think the rust , junk & etc. is clear from the lock , use a silicone spray lubricant (same precautions as above) with the compressed air method (repeat as needed , until no more reside comes out of your lock) .
Do a light 'scrubbing' of your key with fine steel wool , spray the key with the silicone & see if it inserts into the lock - don't force it - and will turn ...you may have to gently rock the key back & forth to get the mechanisim to begin to free-up .... just don't force it , Keith .
If your key does not insert fully , or does not turn , repeat the above procedures -except for the key scrubbing -until it does .
It appears as if there was a hasp installed at one time on your trunk -this might mean that the lock was/is totally broken ... so don't get your hopes too high , as to an easy fix on it ..but, who knows.