Dean;

If its already a 12V system, you have lots of options...limited only by your budget...I do prefer to install a functional vintage OE radio in the normal position for a proper period look, and to add a modern sound system discretely (with equally discretely located speakers...under the dashboard facing passengers as well as possible).

When installing a sound system into a vintage Volvo which did not previously have one, you will need to add RF (Ignition), as well as power line noise suppression, see: http://www.sw-em.com/Radio_Notes.htm ...also be aware of high stand-by currents which cause Battery discharging and have falsely been diagnosed as Battery issues. See: http://sw-em.com/Battery%20Notes.htm#overnight_battery_discharge ...so you might want to power a power stage by way of an Ign Slave Relay. See: http://sw-em.com/Ignition_Slave_Relay.htm

Cheers