1996 Pontiac Grand Prix Question about intake and head gask

I'll try to make this short. After having the infamous dexcool cooling system problems with the cooling system having major blackage, overheating and such. My wifes car had major repairs for the second time (intake, heads, gaskets, sensors etc).

The last time it was done at the dealership about 8 months ago. Originally they only replaced the intake gakets for a bad leak that developed. After bringing the vehicle back 6 times (never got more than 5 miles from the delaership without the car overheating and developing blockage in the coolant system from internal junk coming loose), they finally took their time and told me they removed and cleaned the heads and replaced all sensors and the themostat.

The car was fine until now. IT overheated very badly in a short drive (less than 15 miles) and lost no coolant at all. The radiator never even got hot, yet the motor almost seized and was very hot. I had the dealer tow the vehicle in (suspecting that it would be covered with all of the work they did previously).

To my surprise, the dealer is asking for another $1600 saying it is the rear head that needs removed and that they suspect it is leaking coolant into the engine. Again. I noticed not loss of coolant at the radiator and should the dealer not have pulled both heads last time? I swear that is what they told me they did last time. Now they are saying the only pulled and worked on the front head previoulsy.

Am I wrong in assuming that both heads should have been pulled and cleaned previously? Why would they not have done that (other than the fact that the rear is hard to get to)? Am I justified and expecting that they shuld have?

Also, is this sound like a good diagnosis and what (if anything) should I expect the dealer to take responsibility for? The repairs are covered for 12 month, 12000 miles and it hs been less than that.

Any reply would be greatly appreciated.