If the switch is making contact you should hear the igniter at the heater clicking and the red light should be on until the heater is light.
If not remove the cover and check for 12 volts at the switch. If the switch is open you should have power on one side. Any metal on the trailer body should act as a ground for these checks. If not check the fuses. If the switch is made and you have power on both sides the wire to the heater may be broken or the limit switches may be bad. If you have power on both sides go to the heater and check for voltage at the the limits switches. They are the 2 small round dime size items attached to the side of the heater, red wires. You should have power on all 4 connections. If you have power on only 1,2 or 3, of them the one you do not have power across is bad. If all 4 have power and the heater is not attempting to come on the control may be bad.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles