Yeah, the springs are a problem. Most of the lock parts are made out of either stainless or a non-ferrous alloy. One of the exceptions, unfortunately, are the springs. From what I understand, there are some grades of stainless that are suitable for making springs, but good ol' carbon steel has better spring characteristics. That is, of course, at least until they rust into little unidentifiable nuggets of what used to be springs.
Here's my hatch lock door. That little brown legless rusty lump in the middle of the picture is what's left of the original door spring:
And for a fix, I harvested a couple springs from other door locks and was able to modify one of them to work as a replacement for the original. Here's a pic of some of the options I was working with. Original rusty nugget at the top and two options harvested from other sources at the bottom. I don't remember what the other sources were, but I have good reason to believe they were later Nissan products or from a Honda Civic:
Based on the dates on the pics, it's been almost seven years since I was messing with this stuff, and I didn't do much writing about it at the time. So I have some pics, but the details are unclear at this point. I was definitely able to figure out a replacement option, but I'm not sure which donor spring I used. I have pics where I clipped the long one in half, but I'm not sure if that's what I finally ended up using:
But for moral support... Here's a pic showing that whatever I did worked great! :