put multimeter on volts and check the starter power lead (earth on engine for test) then crank it over and see what the voltage does, if it drops real low or to zero then check battery when cranking if it stays right then there is a connection issue to the starter or poor earth to the engine, can do same test with earth on chassis if volts stays good then but not when earth on engine then there is a poor earth to engine, u could try jumper leads just connect a good earth from chassis to engine to test if that works then u know issue or if not then u know earth to engine is good and must be a power issue to starter