The reference is in Gruss' book The Four Presidents of the Watch Tower Society page 31-32. Gruss is referencing Salter's letter dated April 1, 1937 regarding Rutherford's behavior, re: drinking and lavish lifestyle.
The quote he uses from the letter states:
... among these luxureis I cannot refrain from mentioning the following:
- Not one, but two 16-cylinder cars, one in California and one in New York...
- Your New York apartment, easily worth a rental of $10,000 a year, and its luxurious furnishings
- Your palatial residence on Staten Island, camouflaged as essential to the broadcasting station WBBR,
- ... A Further small place of seclusion in the woods of Staten Island.
- Your further abode at San Diego, for which you yourself told me you were offered $75,000, but it could not be sold and the funds used to help pioneers because it was deeded to Davis [of the Bible] --- what hypocracy!
- Commodius and expensive quarters in Magdeburg, Germany, for the convenience of THE PRESIDENT, to say nothing of provision made for your comfort in London
So the information about the 2 Cadillacs somes from Salter and is quoted by Gruss.
Gruss also asked a New York Realto about the luxury apartment that was leased by Rutherford at a cost of $10,000 a year. Gruss wanted to know what that would cost today.
The Realtor said to forget it. He jokingly said his calculator didn't register that high.
No doubt about it. Rutherford lived the good life at the expense of his followers.