Searching for oxygen sensor?

i was recently told where my oxygen sensor is on my 4.3 .i have 1 more question is it normal for an oxygen sensor to cause a engine to misfire only when it is raining out.i changed my wires,distibutercap,and rotor button and thought i cured the problem.then as soon as it rained again it started missing again.i checked the wiring on the sensors and they apear to look fine but that is what the code sugest.i gust hate to start replacing sensors as some of them are pretty exspensive.