It's a trade off. Heater cores rarely fail in new (10-15 year old) cars, so making it easy to get to is not high on the priority list when others issues are.
The only true fix for your problem is to replace the heater core. Spend the bux and get it done the right way, or buy another car.
As so many others have said in this thread, -any kind- of 12 volt heater simply won't put out enough heat to make a difference. If you're in a reallly cold area, you won't even feel the heat.