l am replacing a failed lighted boat compass with a duplicate. The old one has 2 black wires attached to a disconnect type plug, but l do not know which wire is and which is -. The new compass has the wires labeled. l would like to wire the new compass using the old plugs.
Since l do not know the polarity of the old wires, what will happen if l hook up the wires in reverse? Will it blow the light, fuse or simply not work?
Thx

Either 1 will work>Just hook up>

If the unit is insulated from any metal hull and the batteries are not attached to the hull, then the polarity for the light will not matter unless there are electronics involved. Take a voltmeter and read the old plug for polarity, that is the easiest solution.

you might short out electrical system or just blow fuse out. Best thing to do is to is get a test light and see what wire got power to it.

probably the new one is LED, which polarity matters. hook it up in reverse, the smoke will escape from the LED. It wo not work without its smoke, and it is hard to put smoke back into an LED. [ok, maybe it wo not get smoked]

multimeter set to VDC probe both wires, if it says ''-12.4'' then your black multimeter probe is on the +12V feed. if it says ''+12.4'' then the red one is on it.

If it is a fiberglass boat you can hook them either way. If there are two wires l would assume that is the case. the bulb does not care which direction the power flows.

There is no problem, nothing will happen. If it was important they would have used a black and a white or red wire to let you know.

no worries bud.
if it is a regular incandescent lamp, it will work either way
if it is an LED, it will work in only one direction - it is a diode and only works when the cathode end is at a positive voltage relative to the anode end. if the anode end is a higher relative voltage than the cathode end, the LED goes out [but it should not lose it is life-giving smoke)

Yeah, you probably will blow the fuse, the light might or not be protected.
Gonna need to find the right leads on this one.

not mentioned in the previous , but, after you use your multimeter to figure out polarity, LABEL THOSE WIRES! A little red nail polish goes a long way.