Re: starter soleniod
Start by polishing and tightening all connections as stated. Dont just look at them , make them shine! Inspect all wiring for cuts, nicks, abrasions, or crunching when you flex it, which is a sure sign of corrosion.
Measure the voltage at the starter (+) terminal while somebody turns the key over to "start" - if you get within half a volt or so of the battery voltage(which should be 12.65 volts), the starter is faulty.
Bad voltage at starter (with clean tight connections), measure the voltage at the solenoid small terminal with the yellow/red wire with the key turned over to "start" you should read within a half-volt of battery voltage, otherwise you have a wiring fault between the keyswitch & solenoid. Good voltage at solenoid but bad voltage at starter means faulty solenoid.
Now connect a single jumper cable between the POS battery terminal and directly to the large terminal on the starter. If the starter spins each time you do that, the solenoid is likely the culprit.
Connect another jumper cable to the NEG battery terminal and a good ground (bare bolt head or bracket) on the engine. Now do the same test you did in previously. If the starter now goes ok, you still have a negative battery cable problem or the solenoid is bad.