I think there are several reasons why you don't see the automotive side of this forum with more participation than the other sections.
First, and this has been touched on before, but I'll say it again. It's no big secret that FSM is geared more toward armor, aircraft and the like with a spattering of subjects such as ships and sci-fi tossed in for good measure. I have even noticed in FSM's special issues that the emphasis is on military, ships, and so forth. There is very little automotive coverage there to put it mildly.
Secondly, I think there are many automotive modeling sites, probably more than those dealing with aircraft, armor and so forth. There are even forums dedicated to specific genres of automotive modeling, ie. Nascar, drag racing, street rods, big rigs, etc. You can pretty much find anything you're looking for in a specific forum when it comes to automotive modeling if you take a few minutes and Google what you're looking for.
As for what automotive forums are best, I have seen a growing number of automotive modelers exiting the Scale Auto forums and drifting to other sites to get their fixes. There are a number of reasons, the least of which is the amount of favoritism and butt kissing that goes on between a few select modelers there. The Scale Auto forums are very glique-ish and if you don't fit into the clique, then you won't experience much in the way of encouragement or input from the members there. I also don't agree with the concept that you can't even mention other modeling sites on the SA forums without fear of retribution, up to and including having your membership terminated. I'm of the firm belief there are more than enough forums and model sites on the net that can help make the hobby a better place for everyone. But, every forum has it's rules and as they say, if you don't like the rules, you can take your toys and go play somewhere else, so that's what I choose to do.
I spend time on the Model Cars Message Forum and find that atmosphere to be geared more toward what I enjoy. Gregg, Harry Pri, and the rest of the mods there run a great board and it's a very relaxed and conducive atmosphere to building models.
It can be very discouraging to take the time to post your efforts and questions only to have your input go unanswered or go without comment. Sometimes that "ata boy" is all it takes to keep the fire burning to finish a project or even start another after your current one is finished.
In closing I would just say that you shouldn't expect a whole lot of input reagrding your automotive modeling efforts here. It simply won't happen with the emphasis being out on armor, aircraft, military and other subjects. It may not be meant to be that way, but it is and us automotive modelers simply need to accept that fact and participate in forums that care and share our enthusiasm.
Here is a list of forums that are specifically dedicated to various factions of the automotive hobby:
http://rodsinscale.proboards.com
http://quartermilers.proboards.com
http://pete389.proboards.com
http://mopars.proboards.com
Maybe you'll find one of these forums to be geared toward your building tastes! Hope this helpd. :)