I tested bulbs (and the grounds) by jumpering from the rear light wire to the reverse light on each side. Everything is ok. At any rate, no voltage to the light indicates to me that the problem is upstream. I will check the switch on the transmission case. Thanks, you saved me from tearing the console apart. I'm assuming that to get to the switch I need to go under the car behind the clutch and up high on the driver's side near the linkage to test. How do you replace if it turns out bad?

Thanks.

CSi's were 4 speeds. The switch is in the side of the front case, up high on the driver side. You can reach the connections to test with an ohmmeter.
Are both bulbs good? Did you check the grounds through the tail light housings? These are often the cause of rear light problems.