Splicing into "ignition run" is not particularly accessible, and is NOT fused.
So for tach power, get the fuse panel down, and look at the back.
There are TWO buss strips on one side, one is always hot, one is switched. There should be some tabs there where you can attach addionable connectors. You should use an inline fuse but these normally come with huge fuses. You only need 1/2 amp or 1 amp or so fuse
On one end of the panel, is the "inst" fuse. It is completely separate, and one end will have some orange wires. THIS is the dimmer controlled dash lamp output, and is where you need to hook your tach light
Typical panel below: From the left, the first three fuses are "accessory" and therefore switched with the key. This is what you want for tach power
The next two towards the right are "hot."
The fuse to far right is the "inst" fuse, and the bottom, where the orange comes off, is where you want the light hooked to.
Also, when you run the tach wire in from the engine bay, try to "dress" it with ties along ground, such as the column support. This helps shield the wire a little and helps keep radio noise down. Also, is you splice a toggle switch to ground into the tach wire and mount it inconspicuously, you can create an anti--theft switch. Some guys use the cigarette lighter!!! Push in the lighter, hooked only to the tach wire, and no spark!!