Same thing here.
One option would be to add a custom slider via Winterboard. Most of them block the "slide to unlock" text--I had no idea 20 second lockscreen would show that stupid thing, but I'm always running a lockscreen. LOL
Otherwise, there's a few of free tweaks in Cydia under "autolock" or "disable" that you might try. It doesn't look like any of them have been updated in eons though. One possibility might be "NoLock". I also saw one that specifically disabled autolock only when AirPlay was in use--depending on how you connect to your car's speakers, that might work.
(Note: not endorsing any of these specifically as I haven't tried them personally.)