I have a 98 corolla and have done what you want to do...
I bought a tach at Autozone. A small 2 3/4" Sunpro. It came with all the brackets and hardware to mount it to the "A" pillar.
What you do is look at the instructions that come with the tach so that you know which wires on the tach go to which signal.
1. Your "Ground" wire from the tach can be grounded to any metal on the car (make sure it is nice and tight and also touching BARE metal).
2. Your power wire should be connected to a power source that IS NOT a constant. This means attach the power wire to either ciggerette lighter, ignition or something that only emits power when the key is in the "ON" position this way you don't kill your battery when parked.
3. You need to remove the instrument cluster from the dash and connect the wire that has the tachometer signal. Every car has it even if it didn't come with a tachometer stock. It is a black wire on the plug second from the left (If I remember right) Your best bet of how to tap the wire is to pick up a wire splice (also avaliable at autozone.
Hope all that helps
