Still no answer on the location of the graphic equaliser but somebody with a Fiat 131 has just had a Voxson Indianapolis renovated and posted a couple of useful pictures.
From what he says it seems the Voxson car radio cassettes were a Fiat group fitment.
Any way, here is the web address;
www.131mirafiori.com/smf/index.php?topic=7619.0Cached and there seem to be circuit diagrams on;
www.radiomuseum.org/r/voxson_indianapolis_923.htmlCachedFrank