I think I finally got my car PC setup to resume correctly. The key was to enable "Hybrid suspend" and set the power supply to never turn off. It only takes a few seconds to come back if the power supply has not shut it off. Oddly, a full resume can take me 2.5 minutes, where a cold boot only takes a minute (but the GPS and sometimes the touch-screen don't always come back up on a cold boot).
Unfortunately I've got the M4-ATX power supply, which has 6 mostly useless settings for how long it stays in standby then powers off. I'd like it to be in standby for one day, then hard-off. My choices are 5s, 1m, 15m, 30m, 2hrs and never. You'd think that with a USB interface, it wouldn't be hard to make those times programmable.
I drive my car daily, so there is no chance of the standby computer draining the battery unless I let it sit for a week or more.
I'm really disappointed in the M4-ATX supply- Good potential, terrible software support. It has a USB interface (using a non-standard plug which is not included), but the software is a systray icon that you have to hover over to see the input and output voltages. Then it fades-in with old data, updates the data and fades out in about two seconds.
I don't know why I'm still amazed about crappy software... I guess because more places really get it right now days...