sounds like you shouldn't let friends or family wire your car... lol, j/k man...
Seriously though, it sounds like you may be shorting out somewhere, you will need to have someone (a professional) look over your wires and check your harnesses for issues.
I let NOBODY, except for certified pros and myself, mess with my car. Period.
the celica is a BITCH to work on, especially in the electrical department. Even Tweeter did a half-assed job on my alarm install, I had to go back and fix about three mistakes they made (thank god the wired the remote start correctly!)
For future reference, tachometers usually have four wires:
a ground wire, which should connect to a metal part of the chassis
a power wire (or hot wire) which should be connected to a positive power source that is live when the key is switched to ON or the engine is running. Good source for this is the cig lighter, or off a fuse in the center console.
A tach signal wire, which is connected to a tach signal. I believe you can tap this off the ODB2 plug, or the ECU. Perhaps even from behind the gauge cluster. Our coils are individually connected to the sparkplugs, so no signal wires there. In either case, make sure the tach's cylinder select switch is set to "4"
Lastly, there's usually an illumination wire, which is connected to a wire that has juice in it when your lights are switched on. Once again, the cig lighter is a good source (it has a lamp next to the lighter socket that does this) the dash lighting or other "lights on" sources are good choices. Of course, you can always wire this to power, via a switch, so you can control it's lighting independantly.
last bit of advice: "I installed one on my honda" hardly qualifies him to install it on your celica. they are radically different cars, and he would be lost, unless he worked on a 7th Gen celica before.