I couldn't find any mention of this sensor location in either the 1954 Shop Manual or the 1955 Supplement. Web searches didn't reveal any good information, either.
Found it, courtesy of this post at the Cadillac/LaSalle forum. It's behind the passenger-side head, very near the firewall, and connects to a Dark Green (DG) wire, according to the wiring diagram in the 55 Supplement.