The general rule is that anything that can be construed to cause confusion about the trademark may be pursued.
What constitutes confusion keeps large numbers of lawyers in practice. Most firms will send out a cease and desist letter at the slightest resemblance. Whether it’s worth fighting is totally case-by-case.
In general, again, any company in a business related to automobiles could not use any term that is too similar to Pontiac. However, unrelated businesses in Pontiac, Michigan, can and do use the name. The online Yellow Pages shows dozens of Pontiac named businesses.
Similarly, there are many products named Cadillac, for example, including a Cadillac dog food.
Parody is a legitimate defense in copyright issues, but is less of one when it comes to trademarks. Since graphics are also trademarked, the use of a graphic could be prosecuted in a Pointy Yac parody.
Coke has prosecuted hundreds of similarly-named products. Cola Coca would certainly be prosecutable.
I don’t think there’s a way to answer this question for a particular case not as obvious as that one without asking an intellectual property lawyer.