If you had an equally strong dislike of spiders or snakes (although in those cases we call them phobias which I know this isn't), then your issues would - or certainly should - be respected. Why would this incidence be any different?!
One theory though, does your DH think he's helping you overcome an issue with this particular model of car, and that is why he is repeatedly pushing it. Like "oh, if TheBigZing could only get beyond her issues insert appropriate model, then she may well find grieving for her mum easier" or does he just really like that car?
For the record, I probably wouldn't do it either, and this is from someone who, about a year ago, wanted to spend a thousand quid (that we don't really have. Not broke, but not loaded, IYKWIM) on a model of car that was my mum's last in the model year and colour. When she bought her's in Sept 1996, Mum said there were only 6 being made in that colour that year. DH had to almost tie me down to stop me buying it, and although I was sad at the time, I knew it was a poor choice.
You are not being unreasonable, and if your DH does keep suggesting it for reasons of love, you have to tell him to stop. Grieving can't be quickened by anything - anyone who's ever grieved knows that.
Sorry if any of this seems incoherent - bedtime now.