Can somebody please where the switch is on a elddis caravan to change the water heater heating to gas. ThNkyou

if fan not blowing check fuse relay --if good check control -switch -regulator -fan

There are two possibilities, one is the thermostat. Many people remove them which spoils the whole heating / cooling system. Since you changed the thermostat, I would check the heater valve on the heater which allows the hot water to circulate through the heating system.
It may be necessary to remove the heater box and flush the heater core since you talk about a lot of rust, which could have clogged the heater core and / or the pipes leading to the heater.

have encountered this before turned out to be faulty thermostat and faulty thermostat switch

There should be a vacuum control switch in the heater line going to the heater core that may not be working.
it shuts the water off to the heater core and when heater is turned on the vacuum switch opens the door inside of switch to allow the water to go to the heater core.

The heater needs to be relearned, i cant remember the exact procedure . The dealer can do that for you . I think it is only a half hour charge

Most cars have a cable operated switch under the hood which allows hot coolant to flow into the heater core when the switch is activated. This switch is usually located close to the firewall behind the engine. Look for coolant hoses running to it.

Have someone inside the car change the heater control to heat while you watch the switch. If it's operating correctly, you should be able to see the movement. If this switch does not move, hot coolant is not entering the heater; and you will not get any heat.

sounds like you need a new heater core

240cpa is right.dodge caravan does not have a heater control
valve and coolant constantly crculate in heater core . use the temperature contorol on instrument panel if you notice no change in heat your blend door in heater box is not moving . and as 240cpa mentioned make sure the t stat is not stuck open.

2002 Grand Caravan. The rear heater also has a resistor for the fan speed control.(3 speed) It is located under the rear passenger side plastic cover. You can take off the rear tail gate lower plastic piece and this will let you pull out the side panel. You can reach in and take out the two screws that hold in the resistor and unhook the wiring adapter. You do not have to take off the whole side panel as you can reach in from the back. The resistor unit can be bought at you local dodge dealer.