“Can an employer forbid prayer in the workplace?”
If the employer forbids all prayer, no matter what the religion, then the answer is yes. The law only requires employers to not discriminate on the basis of religion (i.e. show preference for employees practicing one religion over another). It does not require employers to allow religious activity in the workplace.
“Can an employer forbid prayer in one's car in the parking lot?”
If it is the employer’s parking lot, the employer can probably do that, too.