Same as an auto trans, there is a cable that attaches to the reverse shift lever on the trans. That cable runs up to a bracket on the steering column, below the brake booster. Your cable may be binding, or kinked, or have some other reason it is not allowing the shift lever to move the way it should. You can disconnect the cable from under the hood for now, if ya want, and then manually move the lever it is attached to on the column UP, and tie it there. That way, you can at least get the key out. The cable is held on by a small clip. Just remove the clip, slide the end of the cable off of the lock lever, and there ya go.
hth
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"