It is generally thought that statements made in the course of plea bargaining are inadmissable if the deal collapses and the accused later goes to trial.  In fact, Rule 410 of the South Carolina Rules of Evidence (SCRE) (which is substantially similar to its counterpart in the Federal Rules of Evidence(FRE) states that ” . . . a statement made during plea negotiations with a prosecuting authority, even if a guilty plea is not entered or is later withdrawn, is not admissible.”
But can the protections of Rule 410 be waived contractually by the terms of a plea bargaining agreement?
Apparently so.  In the recent case of State v. Wills, decided September 1, 2010, the South Carolina Court of Appeals ruled that the terms of a plea agreement take precedence over the the provisions of SCRE 410.  In that case, defendant Wills was charged with murdering a certain Julian Lee.  Pursuant to a Plea Agreement, Wills agreed to cooperate with the authorities in the investigation of the murder in exchange for a reduced sentence.  The Plea Agreement that Wills signed (which was reviewed in advance by his attorney) stated that:

[U]pon examination(s) by polygraph, if the responses given by Theodore David Wills Jr. show deception, are inconsistent with information previously provided or indicates he is the person or one of the persons that shot the victim, the terms of this proffer are null and void and any statements made by Theodore David Wills Jr. may be used against him by the State for any legal purpose, including, but not limited to, considerations for charging, bond, disposition of charges through plea or trial of Theodore David Wills Jr. and impeachment.

Wills later failed a ploygraph, his statements were used against him at trial and he received a 40 year sentence for his murder conviction.  He appealed claiming his Plea Agreement granted him immunity and citing the provisins of SCRE 410.
Unfortunately, his Plea Agreement contained no “immunity clause”.  Citing other South Carolina case law, the Court of Appeals noted that  “immunity agreements and plea agreements are to be construed in accordance with general contract principles.”  and, therefore, “[t]he court must enforce an unambiguous contract according to its terms, regardless of the contract’s wisdom or folly, or the parties’ failure to guard their rights carefully.”
The Court did note that there was an “ambiguity” in South Carolina law that arises from the conflict between the case law on the interpretation of plea agreements and the provisions of SCRE 410.  The Court also noted that this issue had never been directly addressed previously in South Carolina.  Accordingly, the Court looked to a U.S. Supreme Court case – United States v. Mezzanatto, 513 U.S. 196 (1995) –  that had addressed this issue in the context of FRE 410.  There the Supreme Court held that “absent some affirmative indication that the agreement was entered into unknowingly or involuntarily, an agreement to waive the exclusionary provisions of the plea-statement Rules is valid and enforceable.”  Id. at 210.
The South Carolina Court of Appeals found the U.S. Supreme Court’s reasoning persuasive and applied it in State v. Wills.  Accordingly, unless and until the the Supreme Court of South Carolina visits this issue, it can be assumed that the terms of a valid plea agreement can waive the exclusionary provisions of SCRE 410.