Vengeance

Its very unlikely that the Central Locking Control Unit is fried, rats can do plenty of damage to the electricals of a car if they lived in it for a certain amount of time, speaking form personal experience.
Just inspect all the fuses (there's only about 10-15) and replace them if any are blown.
Inspect the car and check for evidence of rats in places where the rats are likely to have lived in (under the carpets, seats, behind the dashboard etc). They'll definitely leave a bad smell and biting marks in things made out of rubber. If you have sufficient evidence, its highly likely that some of the wires are bitten off.

Remove the negative battery cable before you begin working on the electronics.
Get a multimeter with continuity and check for continuity on the central locking wires, the ground of the battery will connect directly to the CLCU and the positive end will connect via a fuse. Then check the wires going from the CLCU to the 4 actuators starting with the master actuator (driver's door one).
EDIT: If the wires are fine, then its the CLCU you have to check. Its a unit with 2 relays, some resistors, diodes and capacitors for stabalizing the voltage. This is a very basic unit if you know your way around circuits. You can check the relays for a clicking noise to confirm whether they are working when the central lock is being used. Replace any broken components and the central locking should work normally. Keep in mind that some CLCUs have a fuse within the circuit itself.