1999 Volkswagen Jetta

The clicking is the starter solenoid.
The problem could be either that the solenoid is not 'clicking' over well enough to activate the starter. Or the starter motor itself is failing (internal short -made worse by moisture).

One often reliable old trick for an intermittent starter (solenoid especially) is to grab a tool like a tire-iron and give the starter/solenoid unit a couple good thwacks, then try starting!