Can you join any other yahoo group?
The Drummond group, as an example, is easy going and postings are allowed almost instantly on registering. The Centec forum is really poor in that respect, sometimes taking days for posts to appear.
Finding you could access, or not, another forum might indicate whether it was yahoo, in general, that is causing the problem or the particular DW group level.
Remember car defects are never common and yours is always the only one with that particular fault ever to occur! So may be the typical bull excrement reply from a head in the sand oppo, or an honest response. Your evidence suggests the former, I am afraid!