I just installed an ISSPRO boost gauge into my '94 940 like you asked about last SAT. I see what you mean by the wires not matching the instructions. Were you looking at the rheostat or the headlight switch? No matter, what I did was relatively simple but not elegant.

First, decide if you want the gauge lights to dim with the dash lights.

Look at the two switch panel illumination lamp sockets/wires. The one on top of the headlight switch (closest to door) has a WHT & a BLK wire going to it. This is a non-dimming illuminator that keeps the headlight switch fully illuminated whenever the parking/headlights are on, just hook into these 2 wires (parallel) if you want the gauge fully illuminated with no dimming.

If you want the gauge to dim (my choice), look at the other illuminator (closest to the steering wheel). This is a dimming illuminator and has a WHT & a BLU wire feeding it. Hook into these 2 wires (also parallel) if you want the gauge to dim with the dash lights.

Hope this clears things up, Volvo (in 1994 anyway) apparently uses a dimming method whereby a full 12V is fed to one side of the dash bulb. The rheostat then supplies variable voltage to the the other side to set the illumination level. Oddly, the more voltage supplied to the variable side, the dimmer the bulb (because the potential on each side of the bulb decreases with respect to each other). As you decrease the voltage, the potential increases and the bulbs brighten. FUNKY SETUP! I wonder how many other cars use that type design.

Anyway, that's how I did it. As far as where to hook in the gauge hose, I found a splice ready-made in the hose feeding the existing gauge right above the brake pedal. I don't know if it is a standard splice or if they were just switching over hose rolls while assembling my car but things couldn't have been simpler.

By the way, I found that removing the driver's dash speaker (I mounted the gauge on the A-pillar), lighting switch panel and knee bolster & backing made the job much more tolerable. I never needed to lie on my back under the dash!

Next, I'm planning on adding a panel rocker switch to turn the A/F meter ON/OFF, that flashing red light is annoying at less than full throttle.
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Bob Kraushaar '94 945T, '91 940T(back from retirement), '88 240, '84 242T, '94 F-150, '89 560 SL, '68 Shelby GT-500 KR