Assuming it is legal in the first place*, the FCC doesn't care about the specific use it is put to, so moving it from being a real estate advertising transmitter to being something else in a car is OK. They do require that the unit continues to meet the technical requirements, which were met by the builder of the unit (presumably), and that the total length of all the wire coming out of the unit (including antenna, ground lead and transmission lines) not exceed 3 meters in length. As mentioned, you are always subject to the part 15 commandment: "thou shalt not interfere with a licensed service" so choose your AM frequency carefully to not transmit over top of a regular AM station.
* if there is no FCC statement on the label or in the user manual, it is probably not "legal" anyway, in which case your question is moot.