[ant7]

First check battery voltage, especially when trying to crank it.

Sounds like there isn't any power going to the solenoid of the starter, or its too low to engage the solenoid, check the wiring on the starter for corrosion, also could be the ignition switch, [make sure the plug on the back of the ignition switch is pushed on properly] you will need to check the voltage you have at the starter solenoid while someone turns the key to see what voltage you have at the solenoid low current wire [the thin one, yellow]
While your under the car, check the thick ground wire going to the chassis from the gearbox, remove it and clean its contacts, refit using some copper grease!
Ant.