I have a 97 Chrysler lhs, 3.5l, it's overheating, water backing up in resevor, replaced water pump, thermostat
When the coolant backs up into the overflow all the time it may be a sign that one of the head gaskets has gone bad.
Pull all of the spark plugs out of the engine, pressurize the coolant system and wait for a few minutes then crank the engine over and see what spark plug hole the coolant comes out of. If it comes out of a spark plug hole that is the area the gasket leak is and that head would have to be removed and the gasket would have to be inspected with also the cylinder head for any cracks.
If you need more help with the diagnostics get back to me and I can help with more testing
You may have to bleed the cooling system by the bleed valve on the thermostat housing.
Slowly pour in coolant into the reservoir while the bleed valve is open, close the valve when all the air is out.
Repeat procedure if necessary.