Just a quick point...
My greatest success working stations mobile in the afternoon has been on 17M. I get the east coast around 2200Z and very quickly signals QSB toward the west coat and the Pacific. By 0000Z anything left is headed west.
I made my first Mobile HF contact to a station in Maine on 20M. Just recently I made a contact with a Station in Hawaii while I was on the Ladies in 80's run from the top of the trail near Alice. Sure, I was at 11,000' elevation. But I still had to work to get that QSO. He was probably being polite and giving me a 5-9 sig report but he said I was clear.
Keep trying, and always listen when you're in the car or at the radio. That way your mind becomes tuned to who you will hear and where they are from and at what time of day it will work. I never imagined that I would make a QSO with a station in Spain at 2030Z (2:30PM my time) on a Thursday, but I did.
Blue '99 UZJ100 2" Lift, Replicated Outback stackable drawer system, Engle 63 QT, Custom Front Bumper, Gamiviti Shorty roofrack.