Yep, sounds like the vehicles are not to be kept on the lot.
Your options are:
1) gather support and remove the existing board members and replace them with those who will enforce that covenant.
2) (This one is often not thought of) Enforce the covenants yourself through the courts.
Typically, CC&Rs authorize both the Association and any member to enforce the Covenants. The only difference is that a member will need to go through the courts to enforce and the member need not enforce on all violators.
NOTE: Based on what you cited, there is no covenant preventing the parking of any of those vehicles on a public street. Therefore, your Board is likely correct that to enforce the covenant could have the vehicles parked on the streets.
Because the street is public, any restriction for parking on the street will likely need to be written within the deed restrictions (i.e. the CC&Rs) to be enforceable. Otherwise, a member could simply challenge any enforcement of parking the vehicle on the street with the principal of
expressio unius est exclusio alterius. That is to say the omission of a particular covenant or term from a contract reduced to writing shows an intent to exclude it (also see
Sarnir R. Farran, et al. v. Olde Belhaven Towne Owners Association page 4, 2nd full paragraph).
Please note, I am not an attorney and I do not work in the legal profession. I am offering this opinion based on what you have provided, personal experiences and my understanding of any research that I may have done.
If you really want to be sure of any legal issue, seek an opinion from a local attorney versed in the specialty of the situation. In this instance, I would suggest one versed in contract law (as the CC&Rs are essentially a contract between all owners within the development).
Likely not what you wanted to hear. I hope it helps,
Tim