What is the main cause of a volt gauge showing a negative reading when the car is running?

What do you read exactly?
There is no negative range on the gauge.

Valy is correct. If it is a mechanical (d'Arsvonval) meter it will only read negative if it has a zero in the center. If it is digital (say a Fluke 179), and it is reading negative, you have the leads reversed.

Olin

If its the one in the console; just about anything. Because of its location in the "chain" it is basically starved for current. A 15w bulb activated will make it read below 12v.

The wires are backwards. The meter only has graduations for positive voltage. reverse the wires and and you measure negative voltage.

please clarify your question before reversing any wires. . what do you mean 'reads negative' ?? is this a stock meter, reading from 0 up to whatever positive, or does it have a 0 in the middle and one side neg 12 and the other side pos 12??

If you reverse the leads inappropriately, you'll at best peg the needle and that can bend it. . the terminals on the rear of the meter should be labeled

This is when a picture is worth a thousand words. .

OR are you talking about a VOM hand held mult-meter. . If yes, then reverse the leads. . red to positive