I'm installing the guage using the instructions from MKIV.com The directions for wiring the guage to the ashtray light only mention splicing-in two wires: 1) the black ground and 2) the 12v wire. There are a total of three wires coming out of the back of the SPI guage. How do I know which one is the 12v one and what is the remaining one for? The colors of the wires out of the back of the guage are black, yellow and green. Thanks in advance.