My 2004 Grand Cherokee's heating system has packed up. It all sounds fine, the fan makes loud noise as usual but nothing comes out (as if something is blocked). Plus there is a blast of cold air on the passenger side with or without the heating system on, again like something is permanently open or shut. The problem is that all Jeep dealers say it is a minimum of six to seven hours' labour just to diagnose the problem (without actually repairing anything) because the heating system is behind the dash and it all has to be removed. Any ideas?