As Clock said any switch that is not already on a relay if replaced with a toggle switch you will need to put a relay on the circuit, if you don't you will burn out the switch due to constant current running through it. The switch will activate the relay and then the relay puts current to what your switching on. Can't remember what your heater box is like under the dash of an 83 4/4 but MMC used to produce a radio mounting box to fit radios into under it.
Hope that helps a bit.
