and when checking plugs for corrosion, especially ones under or near the battery, look close at the wire itself before the end terminal.
those wires can [and will] turn "green" from the battery outgassing, and although the terminals look clean, the wires can turn green under the insulation.
if you don't look very close, you can miss that. i have rewired many vehicles over the years [of all makes] that have had happen with wire bundles located around the battery.