I forget how the thing is held in on a 940, but you can pick up an
installation kit from Crutchfield that includes a mounting bezel and a
wiring harness to plug right into the factory wiring. A local stereo shop
may stock the same pieces.

If my memory serves me, it has been about 3 years since I pulled a
factory radio from a 92 940, here are the steps.  To remove the factory
radio remove the tuning and volume knobs by pulling them straight out.
Next to the metal shafts are small metal tabs with small rectangular
cut-outs. using small straight edge screwdrivers pull the tabs towards
each other.  This releases the latches on either side of the radio.  You
will need to do this at the same time as you start to pull out the radio.

From there on it is straightforward.  Might I suggest that you use a
prefabricated adapter/installation kit that includes a mounting bezel
and a wiring harness to plug right into the factory wiring.  Such things
are available from Crutchfield and other suppliers.

Depending upon the sound system originally installed there may be wiring
for a remote amplifier and remote CD changer.  The amplifier, if fitted,
exists under the dash aft of the firewall and near the steering column.
 The CD changer, if fitted, would have been fitted in the trunk.

Getting out is the tricky bit, follow the other guys advice, but if you
can't get it goto where you bought the new unit they will be able to do
it, worst case Volvo dealer (try the parts dept, a route to sympathetic
mechanics that know you don't want the car washed and waxed for £50/hr
when it is serviced).

I prefer to rewire it to a standard ISO connector, then it will fit any
radio.  Soldering is a good idea.

I also added a MP3 compatible CD multichanger and attached it to the
roof of my estate just behind the back passsengers head where there is
an upwards recess.    Its very difficult to see from the outside and was
relatively easy to route the wiring along the roof corners.

--
TonyS