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On the back of the tachometer, you'll see a small (roughly) triangular section with three spade terminals. The two along the edge are marked + and -, and these are obviously for the power to the tach. Slightly closer to the center of the housing, midway between the + and - terminals (think "triangle") is a third spade terminal marked G, which will be your input from the coil for your tach signal (more about this below).
Of course, you'll also have your lamp for illumination -- this varies over the years -- the bulb holder may have both terminals, + and -, or just + with the case as the ground. For the +, just tap the ubiquitous brown wires that carry current to all the illumination bulbs behind the dash.
As for the tach signal input, you have an '89, and sometime between '84 and '93 (the model years of my cars) Volvo added a tach input wire (white with red stripes) behind the main instrument cluster. If it's there, you can use that -- otherwise, you'll have to run a wire through the firewall/bulkhead from your coil to the tach.
Of course, you'll also have to get the bezel, and you should get the three little rubber pieces that hold the tach (or any 2-1/16 inch gauge) in place in its hole in the dash.
It's a lot easier than all this sounds if you're handy with wiring connectors, etc. The only hard part is if you have to run the wire from the coil to your tach.
Good luck.
Here's my '93's dash, by the way (with custom gauge faces):
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