30 (other red wire) needs constant 12-volt power from the battery. 86 (gray/white wire) goes to the ignition switch. 85 (black wire) connects to the temperature-controlled sending unit.

A 12-volt, NO/NC, SPDT version like this one (Hella PN 007794301, $6.38 at Summit) can control both normally open and normally closed circuits, or it can act as a changeover device to switch ...