Where is the speed sensor on a 1990 Cadillac deville?

The speed sensor on a 1990 Cadillac DeVille is located in the transmission. It's not easily accessible without removing the transmission or at least partially disassembling the transmission's bellhousing. There's no single definitive answer on exactly *where* in the transmission it's located without a repair manual specific to that year and model. It's a sensor that integrates with the transmission's internal workings. You'll need a repair manual or a trustworthy diagram to locate it precisely.