The fuse box is not in the engine compartment. Check below the radio, behind the garbage container.
you can find it in a workshop manual
http://www.volvoclub.org.uk/faq/WaterPumpChange.html
The Chiltons/Haynes 72300 has the vacuum circuits divided into the emissions, Fuel and brakes sections of their manual covering each vacuum hose.
infolinks.com free pdf diagrams... took me forever to find these... be blessed
The engine is not running!
The ignition relay on a 1992 Volvo 740 Turbo wagon is typically located in the engine compartment, near the battery. It is mounted on the inner fender on the driver's side, usually within a relay box. To access it, you may need to remove the cover of the relay box. Always refer to the vehicle's service manual for precise location and identification.
In a 1992 Volvo 740, the fuse for the horn is typically located in the fuse box under the dashboard on the driver's side. You can find the fuse box by looking to the left of the steering column. The horn fuse is usually labeled, so refer to the diagram on the fuse box cover for identification. If the fuse is blown, replacing it should restore horn functionality.
Haynes Repair Manual for Volvo 760/740 has diagrams of the vacuum sub-systems.e,g. the EGR, the ignition etc
On a 240 Volvo (which basically should have the same engine) it is directly under the intake manifold. On a 1985-740 previous comment is right On a 1990-740, driver side, on top the engine between injectors #3 and #4 a big tubing and a small vaccum hose attach to the flame trap
You need to replace it. There are 2 screws that hold it on.
Remove the ashtray. Then lift up and out on the tab labeled "Fuse Box"