dennisafrompa

.....why is there one in there as well as the readily accessible one in the rear trunk?

Porschetech3

The only logical reason I can give is, the easy access one is for checking, topping up, the one in the engine compartment is for pressure vent and for complete refill after repair ( it's threads fit the already available evacuation tool ).

MrMoose

The cap in the rear trunk is just a seal. The cap in the engine compartment is vented to limit the maximum cooling system pressure. You need a vented cap somewhere in the system.

If you only had the vented cap in the rear trunk, you'd get a coolant smell inside the car as the cap did its job. So they stuck the vented cap in the engine compartment and put a more easily-accessible non-vented cap in the trunk for filling.