Hello and thank you for your message.
Without seeing the suction cup holder and the heater unfortunately I'm not sure what would be the best way to remove it.
Depending on the type of heater it is, some types of glass are more shatter resistant so you may be able to use a screwdriver and gently tap the screwdriver with a hammer to break the holder free of the heater. There is no way I would be able to guarantee this would not break the heater though.
I apologize we aren't able to give better solutions to this problem.
Marine Depot Customer Service