I used a cable release, your seeing the cars vibration in my shots. At times like this, I miss my Caddy

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I do agree it would work for the cars interior, the car and camera are vibrating in tune with each other. I could also use a small flash pop to burn in the interior.
I used much shorter times, I shot at 1/4 sec, f5.6 ISO 400 for the first, and 2 sec, f5.6 ISO 200 for the second. I have a few that are sharper, just haven't had time to work on them. There is an element of chance to it too because, the entire platform (the car) is moving. For outside shots I am going to try something hand held with the counter weight, short-ish exposure around 1/4 sec, and with active VR. I am hoping it will keep the center somewhat sharp and let the motion blur happen on the edges...
For the interiors, I am going to go with your recipe (it works) but I may try a small pop of colored strobe at the end of the exposure. I am also going to make sure the windshield is spotless. When my exposures went over 2 sec, the dirt ruined the effect for me.