I can't tell you exactly, could be electrical or physical problem (there
must be a solenoid or small motor that works off the door lock system).
If you open the trunk and remove the upholstery from the left (driver)
side, you should be able to see the mechanism and maybe you can figure
out a way to disengage it. You can probably troubleshoot it while you
have access to it, too, by disconnecting the harness and applying 12v
and ground to the appropriate pins in the connector. If the harness
connector is wet or the pins are corroded, perhaps the electrical signal
is not getting through to disengage the lock; or, perhaps there's rust
on something in the mechanism that slides or pivots.

I. too, have problems when my 89 sits on the rain, like a trunk full of
water and an engine that won't start. I'm working on the starting
problem (ignition, I'm fairly certain), but I still haven't figured out
how the water gets in the trunk.

Something else to check:  if I'm reading the manual correctly, fuse 13
in the left fuse block (under the driver-side dash?) provides current to
the left rear door as well as the fuel filler flap lock, so you might
see if that fuse is blown.

Open the trunk,  pull back the carpet under the petrol filler and you will
see a small white push button/lever.  Push this up and the lid should pop
open.  Make sure you check to see that the hose line running down the side
of the filler tube remains connected or you will end up with a trunk full of
water. It may be a good idea to check to make sure this is clear as well.

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