Depends on how the first aftermarket radio was wired in.

The factory radio came with a separate amp, tucked behind the lower console. That's what the 6 pin radial connector was for.

Installing an aftermarket unit required that amp to be bypassed, either with an adapter harness, or by hardwiring the new radio in. Take a look at what you have now. If you can see two rectangular connectors, then the Pioneer was proably installed with an adapter harness. If all you see is loose wires, then it was hardwired. Most reputable shops used harnesses, which were available from Metra.

If you have the connectors, and the harness that came with the Sony, you might be in luck - the wiring/connectors may match up. Sometimes, everything matches except the speaker sides are reversed. If not, you may be able to get adapter harnesses from Crutchfield.com by providing them with the model number of the Sony.

If you need to hardwire, here is a wiring diagram - on page twelve of the pdf file.

http://pliki.volvo.techno.org.pl/schematy/volvo_240__1992.pdf

Or, if you want to upgrade, Crutchfield has some good deals on decent units, ipod ready, for under $150, and they throw in the adapters, harnesses, instructions and shipping. I've done three upgrades with gear from Crutchfield in the past year, each was a piece of cake.