The 2012 pack heater is driven by a relay, Panasonic AEV 52012.
One of the HV lines is switched by the relay, the other line is a direct wire thru the connectors with a 10 Amp inline fuse rated 500V, here is the
fuse datasheet
The module also holds a separate control board that receives the signals and sends the coil drive to the relay. This board is enclosed in a metal box which is soldered together top and bottom.