I have a 92 SC400 and My Trunk won’t lock when I electronically open it from the driver’s seat. If I open it with the key it will lock when I close it. So I am thinking it might be an electrical problem. It’s really annoying and ideas for a cheap fix will be much appreciated.

 

It's not an electrical problem. It's mechanical.
There is a plastic cog inside the locking mechanism that is supposed to reset the latch when you open the trunk from inside the car. When the plastic god gets worn, it doesn't return the latch into the position it should be in for it to lock the trunk when you close it.

When you use the key to open the trunk, it operates a different action all together to release the latch.
This is why the trunk will not close only when you open it from inside the car.

I do not know if you can actually replace the plastic cog, or repair it...
I was able to get a used lock from the local scrap yard.
I was also able to get it free, so I don't really know how much it would cost either.

If you take the lock apart, you can see how it works and then have a good understanding of how that little cog could make your life a bit difficult when it gets worn.

Hope this helps.

Carib.