Air/Fuel ratio gauges

I have a question for the people that have put A/F gauges on their cars (if anyone has). I have an 89 with the 4 wire 02's, which one of the 4 wires is the signal wire?? Any help would be appreciated.

If I'm not mistaken, you have to tap into one of the TWO wires after the 4 wire connector. I think it was blue.

Thanks for the input, but is there any way I can test that b4 I go cuttin into wires??

I made a small slice in one, hooked it up and tested it. It didn't work so I tapped it up well and did the same to the other and it worked. Don't take off a ton of wire, just enough to make contact.

I'd check for you, but my car is too damn low to get under it