Is it the standard Sony headunit you have fitted?
When they were new they were sold with a flat 'key' which fits into two slots within the fascia (face plate removed), one on either side.
If you no-longer have the key you can use all sorts of impliments, (Spark plug gappers work quite well) including strips of a cut-down credit card.
If you're using your improvised key correctly no force should be necessary, it'll just pop right out.
If all else fails, pop down to your local stereo installation shop and ask nicely - I'm sure they'll release the old one for you. Go in on a sturday when they're busy, give them some schpiel, tell them you bought a new headunit from them last weekend, and can they release the old one for you. They'll do it for no charge in a matter of seconds...
