1994 Ford Explorer Won't start, clicks

I have had intermittent problems recently when starting my 94, with just a click when engaging the key. Tightening terminals, cleaning them, twisting them etc., would usually cause the starter to engage. Recently I had the alternator go bad, so I changed out the battery and the alternator (the battery was old). I thought that this may have been at the root of the problem but this latest incident will not resolve with any of the previous methods. Last night I changed out the starter relay switch. It still clicks. Should go after the starter next or look more closely at switching out my cable leads to the relay from the battery. The ends that conect to the positive terminal look to be worn. Could corrosion inside the insulation be the issue?