If you see where the cruise control connects (try to look at another car if yours has no cruise) you'll see how Volvo ran extra vacuum hoses. They used a small diameter black plastic T fitting and a short length of hose.

As noted, it needs to be on the manifold side of any check valves. You may be able to use windshield washer tubing from a junkyard car if you have trouble finding the right size small vacuum line.

The turbo gauge will spend all its time over to the left with the engine running. Straight up and down is zero vacuum, i.e. lowest efficiency for a non-turbo engine. Expect the gauge to approach that point on brisk acceleration. The further over to the right you can keep the indication, the more efficient you're being with fuel. Mine, for instance, would likely be straight up and down most of the time. :)
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Rob Bareiss, New London CT ::: 87 244DL, 88 245DL, 90 745GL, 84 242DL project, 89 244 parts, 88 244DL to replace the 87

Additional to the good info from Mapleleafer -

If you choose to "T" into the vacuum hose supplying the cabin air flow control system, be aware that that hose has a check valve in it. For your gauge to work correctly, you must tap into the hose ahead of the check valve.

Should be pretty easy. A small diameter hose will do fine. If the gauge is lighted, side tap into any of the brown leads feeding the various lights in the control panel.

Good Luck

Bob

:>)