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Tom,
Maybe this helps (I just changed my rear pick-up this weekend). The wires should come off the mag on starboard side of aircraft - it is the vent hole on the half of the mag closest to the engine, inboard (between the two mags). The ground wire (yellow in the picture) likely goes to a screw in the cover of the same mag, on a small terminal end. The signal wire should come down over the alternator and into the bundle on the firewall (blue in my picture). Sorry about this picture - it is not from this weekend, but rather a very old picture from when I started taking this aircraft apart to rehab it. There is likely one of the Cessna quick disconnects on the signal wire into the bundle.
My tach initially was working, then one day it swung wildly, then a few day later it just gave up. I trouble shot all the easy things first, swapped leads at gauge to see if it was gauge, changed out the box...finally, checked the wiring from the sensor/pick-up. Only thing left was the pick-up. Yes, it is tough to get your hands in there. I went through the left side cowl, and I had the top cowl removed as well as I was doing other work (i advise to do the same. I removed the starter - and disconnected the wires, and fed them up into the area between the mags. I used a stubby 3/4 box end wrench (fed the wires through the box end) and you have just a enough room with the wrench facing fwd, to slide it in and your hand, and was able to get on the sensor and break it free. Do the opposite if you have to reinstall new one. I tried a bunch of other methods, but that seemed the easiest. Could probably remove the vacuum pump to make it easier...but I was lazy at this point. You know what the real pisser is? First flight after the annual yesterday, and my rear vacuum pump failed. So now I have to go and remove that bugger tomorrow anyway. So, my final advice for you is - if you are going to have replace that pick-up for the tach, honestly, buy a new vacuum pump and change it at the same time!!! I am totally serious.
Jeff
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