i need help on hookin up a after market tach on my 1990 toyota camry. it an auto and im not sure which wires to hook it up to

I hooked an aftermarket up to mine, all you have to do is ground the ground, then I hooked the power up to my headlights by putting the actual wire between the fuse and fuse holder in the fuse box in the engine. I did this because that way the back round light would only go on at night. If you have a shift light then just plug it into something that is on all the time. Ok now for that wire that goes to the engine, if your car has not had a tach previuosly, there should be a plug that is capped off around the area below your distributor cap, take the cap off and just put the wire into the plug. it's easier than you think, if there is more than one capped off plug do some tests.