J.C.
Simple. Stop making your husband talk to his Ex! He is not your child, he's your husband. He doesn't want to talk to her and from what you've said nothing good comes of it when he does. So stop!
Your son is 16, you're just going to have to raise him the best you can when he's with you. You can't worry about what she's going to do. You have no control over that no matter how much you want it. If you don't feel comfortable with your son taking the truck to his mom's house, then tell him the truck is for his use when he's at your house. If he gets into an accident and there is a bill to be paid you and your husband are going to pay it, if you can't accept that then get rid of the truck.
Bottom line is that things aren't going to go your way. Your husband isn't going to get on the line and tell off his ex. That doesn't mean he's in love with her. His ex isn't going to all-of-a-sudden decide to start openly communicating with you about your son and raising him the way you want.
Once you accept these things and let them go you will be free to have a much more loving, peaceful relationship with your husband.