Head has to come off for guides to be pressed out and replaced. There is a drain plug in the radiator, left lower corner facing the car. Believe it's red in color. Cool on the highway with a low ambient temp is not abnormal. If it's hot in traffic, even with low ambient, start looking at the t-stat, have the radiator flushed when you are having the head done. If the coolant was rusty as you describe, have the tech check the water jacket openings in the deck when the head is off, you may have to really flush all that crap out. if you remove the t-stat, take a look at it, if it's gummy and full of crud, then, it's probably kaput.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE