If the locks go out 'all at once' I'd put my money on the key. Order new ones from the local dealer, it's pretty fast and not too expensive. That will also help extend the life of the locks, if done regularly. We did it with ours at the first sign of trouble. I also installed keyless entry, further to reduce the lock/key wear and tear.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7