Does it just click once, or clickety clickety .....bzzzzz.....?
One click might be bad main cable to starter, bad connection at starter stud, or bad starter
Bzzz usually bad battery, cables, connections
If it does this fairly "regular" "rig" yourself a wire from the main big battery stud (clip lead or bolt on) up to where you can read a meter, and see what you have for voltage there at the starter.
What's your wiring scheme? Truck/ front mount? How you have the cables wired/ routed?
