Patricia Spence, who has been diagnosed with terminal cancer, said she was desperate for any information that might lead to police discovering Lynda's remains so that her family can lay her to rest.
Colin Coats, 43, and Philip Wade, 43, were sentenced to life imprisonment for the murder of Lynda, who disappeared in April 2011.
The 27-year-old, who had been recruited as a police informer, was abducted and held at a flat in West Kilbride, Ayrshire, where she was tied to a chair, burned with an iron, had a thumb cut off and toes crushed. Two other men admitted assault and gave evidence against the murderers.
Mrs Spence, 57, yesterday pleaded for information on her daughter's death, saying she had been given two years, "maybe less", to live after her cancer diagnosis.
She said: "If you have any heart at all, any conscience, just tell me where she is and give me her back. I'm asking you and I'm pleading with you. Give me Lynda back before anything happens to me.
"Lynda was my only child. It would mean everything to me to be able to lay her to rest. Without her body there can be no closure."
Coats and Wade are both trying to overturn their murder convictions and part of their appeal is based on a claim that police withheld information about Ms Spence's work for them until it was too late to be used by defence lawyers at the murder trial.
After a hearing at the Court of Criminal Appeal in Edinburgh yesterday, a senior lawyer will be allowed to inspect files relating to the case.
Coats and Wade had made a previous plea to appeal judges to order the police to open their files.
A further hearing is scheduled for next month in Glasgow.