Teacher, Pete, how do you get your spouses to buy-in? My hubby's got a homemade FSK, wool blanket and a homemade water bottle PSK kit in the back of his truck, along with a bunch of vehicle repair stuff (he's a parts guy by trade) but if I put anything else in, it always ends up being taken out.
You didn't ask me, but I'll give you my $.02 anyway.
Time.
Time and time again, my wife needed something that I had in my pockets or in my kit. She wasn't thrilled with me putting a kit in her car, but grudgingly acquiesced. And she found that she was using things in it and glad to have them. When she took her Girl Scout Troop camping and was almost constantly using things out of the kit I sent along with her, she started to really buy in.
It sounds like your husband already has some good stuff in his car and believes in preparedness. Do you talk with him about the reasons for adding other bits of kit?