Dated August 1937, it was found between tiles during a kitchen renovation at Katie Tiplady-Startin's terraced home in Coundon, Coventry.
Gordon Nettleton - the man it was addressed to - has now died.
But Mrs Tiplady-Startin has been able to find his son Tim using a combination of Facebook and the genealogy website Ancestry.
Speaking about the moment she heard from Tim, she said: "It was really exciting.
"It was Sunday and this name popped up in my email and I thought 'finally I have found him'."
George Nettleton lived in the house in the 1930s and the sender, his cousin Martin, was planning a visit to see him.
The postcard - featuring a half penny stamp of King George - shows Martin in his car on the front.
It says: "I expect to leave Wetherby about 10am on Monday 9th... as the distances is ... miles I should reach you by 2pm.
"I approach on the Netherton Road and will ask a bobby the way to your house.
"This is my car in the garden of my house where I now am until the 7th, Martin"
Mrs Tiplady-Startin plans to send the card by recorded delivery to Tim Nettleton's home in Switzerland.
Tim Nettleton said it was a "wonderful find"  and he could remember visiting the home when he was young.
He said: "The Nettleton's moved to Coventry in the '30s from Yorkshire and bought the home in 1933.
"I have no idea how the postcard got into the wall. I guess simply propped on the tiles and there was a gap."
A postcard is going to be reunited with the family it was intended for almost 80 years after it was sent.