Stick with a backpack or see if there is a way to integrate something into the Frontier. For example, a Domke F1 bag could hold (not gracefully) two bodies, three L-series lenses and a flash and would fit in the center console of a Suburban -- perfect for grab shots or easy to pull out and walk a short distance.
Vests (Domke and the like) are nice if you are going to be standing around a lot or need quick access to light weight gear, but I would not drive with one and they are not easy to put on and take off when loaded.
I like belt systems if I am moving fast, but mine is too big so if anyone wants a size large Kinesis system let's talk. If you go the belt route I really like Kinesis -- it is a mom and pop shop and they are great people.
At the end of the day I use LowePro bags -- Mini Trekker if it is just the camera and Photo Trekker if the laptop is coming too. To be honest I really don't have a lot to recommend either of these bags, but for car transportation they protect the gear well. I would seriously suggest checking into LowePro's Omni series. They are padded, have straps and fit inside a Pelican case.
What I do, which I would never advocate, is have most of the gear in my bag of choice accessible in the back seat area. One or two bodies and two or three lenses in the front seat -- either my lap or passenger floor. Despite how hard I try otherwise, meter, film and/or cards and polarizer go in pockets of my shorts (meter usually around my neck), one body with mid range zoom, one body with 180, wide angle lens in pouch on floor or easily accessible in bag, tripod as extended as possible across back seat floor.
And for the record, a vest makes you a dork (or a nature or travel photographer, which are subsets of dork), a belt system makes you paparazzi, and either one with a telephoto lens -- let's say any lens longer than 8" with a hood -- makes you a stalker. Poorly designed bags (like LowePro) make you a naturalists, because as soon as you put the bag down and back on, whatever the bag was resting on (mud, pine needles, dog poop) will be ground into your clothing.