Pull out the 4-wire connector and turn the key into position "2". Line #1 should be +12v. The voltage comes from the fuse box, possibly fuse #12.
Line #4 is ground. You need to check this wire for good ground too.
Line #3 goes to the lamp (If you bridge line #1 and #3 with key into "2", the indicator lamp should be lighted up).
Line #2 is from the starter.