Chris,
The usual thing is for the radio to slide into and lock with the surround which is itself located into the dash aperture by tabs which are bent into place behind the dash. The removal keys which you have should allow the radio to be released from the surround and the surround itself should then be removeable once you have located and straightened the locking tabs.
If your 964 is the early type with the open under-dash area it's not so bad since you can fairly easily get your hand up behind the radio. With the later type which has the leg protector under the dash it's a bit more difficult.
Depending on the model of radio I've found that you often have to remove the outer bezel before you can insert the releasing keys.
Hope that helps.
Regards
Dave