You can try M1 adhesive remover. www.goodbuyguys.com under adhesive removers. Masking tape is more or less a wax type paper with glue adhesive. Good chance that adhesive is pretty dried out by now. You can first try laying rags soaked in HOT water to soften and saturate the tape and loosen the adhesive. This may or may not work depending on the type of masking tape it is. Even if this saturates the paper part of the tape and exposes the adhesive your ahead of the game. It will allow the adhesive to become somewhat gummy again possibly but then use your adhesive remover to remove the glue and residue. You can try it on a small area and see how it works.