Depending on the time of year, the requisite temperature range of sleeping bags are also in the trunk.
Excellent! I keep mine in waterproof bags to keep them dry. Dry bags are durable, versatile and pretty light.
There is a post I made last year here that listed the typical contents that are always in the car trunk. I'll see if I can search for the post and link it here.
Awesome! I like redundancies in my essential gear but not my responses.

Sorry if I'm being too basic.