What is it?
 

After owning it for a year and a half I'm still making "discoveries" on my MkII 960 [an early one was no dipstick for the auto box] todays "find" is what appears to be a cylindrical device mounted behind the passenger side headlamp[and a bit lower so it sits at a height below the black plastic bulb cover]and ahead of the air filter box.It has what look to be two vacuum pipes and a larger bore rubber pipe going to the top of it.Any ideas what it is and what it's for?

A phot would be helpful but from the description sounds like maybe a cruise control actuator. Could also be the electric headlamp aim adjuster to lower the dipped beams when carrying a heavy load so you don't dazzle oncoming traffic.

The air blower to heat the cat

I'll see what I can do re-photos but I'm not very good[OK totally hopeless] when it comes to working out computer stuff.

It is probably the carbon cannister for the EVAP system

The air blower is on the drivers side https://www.volvoforums.org.uk/showthread.php?t=311778

It will be carbon canister then.

If it's on the LHS then that's exactly what it is.

Carbon canister it is then.thanks chaps another mystery solved.

There are two thin plastic pipes connected to the top cap. The first is indeed for vacuum which controls the outlet valve that releases vapour from the canister only when the engine is running. The second one is to carry that vapour into the throttle body. The rubber hose connects the canister inlet to the fuel tank breather.

Electric vacuum pump for cruise control is mounted on the inner wing/wheel arch in line with the strut tower. Electric headlight adjusters are indeed in the same area to the EVAP canister but are rectangular and mounted directly to the back of the lamp casing whereas the canister sits lower down in the "battery" tray.