Use small computer speakers (or anything small) for additional checking at the mixing room. They get overloaded by bass pretty easily. Tweak the mix so that you can hear the bass without saturate these small speakers, while verifying with the main ones that it's balanced. Then it should sound fine in the car.
If you can't avoid saturation in the small speakers without breaking the balance, raise the bass cutoff frequency in critical bass tracks and/or more compression in these tracks or more bass compression in the mix with multiband compressor.
If still have problems, you may try adding some distortion to the bass tracks, or use one of these plugings that synthesize bass harmonics. In either case it should be before the highpass filter.
Take your time with this, it is not easy at first, and even less easy if the mixing room is not very good. But after you success a few times, you will find that you foresee potential problems even before checking with the small speakers and know how to fix them quickly.
 

Yeah, car stereos are great for one thing mainly. I listen to stuff like Audioslave, Rage against the machine, Linkin Park, etc in the car. That kind of stuff sounds great cranked up with the windows down driving fast.

I added a new head unit, speakers, sub, crossovers, amps for rocking out while I drive. I never listen to mixes in my car though, sounds like crap. I don't play music like that personally. I do find that my wife's minivan is very telling.

I guess the point I'm trying to get to is that a good studio with a neutral room reponse is going to sound different than a Aiwa boombox in a concrete basement...as is a car stereo optimized for one sound in a certain car, yada yada.

I vote you ditch the sub and get some good cans to refernce the bass. It's way cheaper than dealing with the room problems you most likely have. I am not a big fan of the auralex stuff. You would problay be better served with the OC 703 stuff and some DIY panels set-up properly. The Auralex treatments I have generally seen only take care of flutter echo which seems like an improvement in the room, but actually only treats a narrow frequency band (2.75Khz to 7Khz). This can affect reverb use in the mix, and cause scouped mids. I have been mixing in my room for about 2 years, and it's an ever evolving project for sure. I ruined many mixes of good songs while learning the room. I add new treatments as I go.

Another thought... might not be a monitoring problem per se, might be you're trying to process the bass guitar too much.   I've seen this problem crop up with too many plugins pumping up this and fluffing out that and I found my bass tightened up a great deal when I trimmed my fx down to only the Ampeg SVX plugin, and applied compression with hardware on the way in.  I'm not a fan of software compression with bass, in general.