Tachometer in a 74 Dart Swinger

I am a rookie at this so I need all the help I can get. I put in all new aftermarket guages and they all work except the tach light. Where in the HXLL do I hook the white wire up to! There are no dash lights to splice into. Someone please let me know what you have done to cure this problem. I wired this entire car with no other problems and maybe I am just having a mental block. Either way thanks for the help!