Bulkhead. Pull a batt cable/seperate the bulkhead & clean the male ones with a metal mini bristle brush & the female ones with a spare male one or a new one from NAPA (725147-99 cents, might be 725145). Then spray everything clean with brake kleen. You might call an electronic repair shop or see what the guys here say for some conductivity paste to brush on, to enhance conductivity/reduce corrosion (I ain't used anything but guys swear by using something). when going back together get em barely started together so you know the terminals are lined up right then press em together.