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was just wondering what the switch with the glowplug simble on it is for in a 1990 bighorn?

Fast warm up system. You will be needing it down there. When the switch is activated and the coolant is below 5C this activates a flap in the exhaust causing more exhaust heat to remain in the engine warming it up quicker. The car will be guttless with it in operation but your heater will of started working before you get to work.
Its quite common for the flap in the exhaust to be siezed open in which case all it will do is turn on a little light

They can also play up and jam in the "restrict" position. Vehicle gets gutless and blows black smoke when it happens.

I have removed or disabled this system in two of my Mu's and will do the same to the other one when time allows.