What, the police car stunk of cannibis??? I'd imagine that, that could that have been a residual smell from a previous arrest they had made though, or a confiscation. In terms of the not having had your Miranda warning I really don't know, plus it is one thing that they didn't say it to you, it is another thing proving it! Also, doesn't it just mean that if they don't read your Miranda warning (i.e. you have the right to remain silent), then any statements you have made under questioning are inadmissable in court?
The evidence in a DD case is simply the breath test reading so they wouln't be using your verbal statements anyway (assuming you were even questioned - most aren't). The only way I can think that this might be a loophole for you is that unless there was a question mark concerning whether or not you were actually driving and you admitted it under interrogation. I might be wrong here and I'm sure if I am someone will correct me.
Good luck though