Complex sentence: The covers to the pickups were stainless steel, which is non-magnetic and therefore won't cause eddy currents, which oppose the magnetic field of a pickup, causing it to sound weaker.
Split into simpler sentences: The covers to the pickups were stainless steel, which is non-magnetic and therefore won't cause eddy currents. Eddy currents will oppose the magnetic field of a pickup, causing it to sound weaker.