Can you be a little more specific on "constant problems with heat in the cab"? Is the problem you get no heat (possible bad thermostat) or it varies up and down from hot to cold without you adjusting the temperature (possible coolant passage blockage, and reason for flushing). It sounds to me (blocked rad, blocked heater core) like someone put Stop Leak or something like that in the cooling system.
Your dealer is telling you what you need to do, but needs to tell you why. Ask the dealer why they think you need a new rad and heater core, and ask them to explain to you how that could be related to a cleaning unit.
Go to an independant rad shop, have them evaluate the vehicle and explain everything to you. Ask them if the failure could possibly be linked to the cleaning unit. Armed with that information, go back to your dealer and challenge them on this.
3,000 kms on the truck indicates this was a pre-existing condition, and should be covered on warranty.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)