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The citrus cleaner I used is DSR-5 Silicone and Adhesive Remover. I believe I bought it from a local industrial cleaning supply company, but cannot remember. It is available on Amazon.
I can't remember what I paid, but it was probably around $25 or 30 a bottle (32 oz.) since I picked it up locally. That sounds like a lot until you realize I've only used about six of the thirty ounces. That includes using it to remove silicone in showers and countertops as well as removing every bit of mastic from my car - and there was a LOT of mastic there.
Be patient, the stuff can only penetrate 'so far' if the mastic is thick. Keep working it in and loosening it up. I used a small putty knife and a firm plastic brush (about 1.5" x 3.0").
And no 'post removal' headache like you get with most solvents.
