P-channel MOSFET for 100A?
I'm wondering what I did wrong


What you did wrong was not understand the peak voltages that can come from an alternator and how these will easily exceed the gate-source maximum voltage for that device (+/- 20 volts).

You should have used a 15 volt zener protection diode across the gate-source and a small collector resistor to limit current into the zener when the alternator produces spikes. The BC337 is also a risky device to use given that it's peak voltage spec is only 45 volts.

Automotive circuits like this are really difficult to get reliable without going for 100 volt rated devices.