Can I suggest a strongly worded e-mail or letter which is copied to, at least:
1. The manager of the dealership.
2. Motability.
3. The head office of the makers.
Your email/letter should include emphasising that due to disability (as using the Motability scheme requires) you required a reasonable adjustment and are amazed that they have refused to make any adjustment as the law on disability requires.
It is still, sadly, the case that many people believe that treating everyone the same means there isn’t discrimination.
Hope you get some satisfaction in the end!