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This sounds a lot like a Folding Top or Front Tonneau sensor going out of tolerance and stopping the cycle since it doesn't know the position of a top component. As a result, the Rear Tonneau won't move. This isn't the only possibility, but it's a fair shot for a first look.
When a Folding Top component fails to move, the problem can often be traced to the component that should have moved right before it. In the majority of the top's electrical problems, the sensors are at fault.
The hydraulic motor will continue to run, but it will be supplying the piping to the last component that worked. Hydraulic supply is controlled via electric valves. If the Folding Top Control Switch is continuously held, the pump will eventually be commanded to stop.
The quickest way to find out is ask your local dealer to give the car a diagnostic scan (this takes less than ten minutes) which should point to the faulty component quickly and reliably. At that point you'll know if you have a bad switch, sensor, pump, valve or control module. All are electrically controlled.
You can buy sensors and install them yourself, but if they have tolerances that are significantly different than the sensor they are replacing, the system may have difficulty recognizing them.
When a position sensor (there are three, plus five switches) is replaced, the folding top must go through a Learning Cycle.
As the top is moved from one stow position to the other, and back again, (one complete cycle) the sensor values (whenever they are attached to something that moves) change. That change is recorded by the Folding Top Control Module during the Learning Cycle.
If one of the sensors malfunctions, it's value doesn't match up with the stored value, so the "brain" halts the cycle to minimize potential damage to the Folding Top. Usually, it will give off a message confirrming what you already know -the top has issues.
To do the job quickly, you need parts, manuals and test equipment. Otherwise, you would have to measure voltage, resistance or continuity by disassembling connectors. Schematics are a must.
This is why GM uses electronic diagnostic tools. The car has info moving though it on two different networks. That info is used to measure the performance of all of the XLR's systems and greatly simplifies troubleshooting.
Top repairs aren't always horribly expensive, so going to the dealer for a diagnosis might be a great opportunity for some what's next/new Cadillac brochure reading time without breaking the bank.
Some of the new user interface displays available (XTS) are amazing!
In the meantime, raise the rear decklid, and inspect the left/rear storage compartment where a lot of the roof control components reside.
1) Inspect the lower drain tube first and make sure it isn't plugged.
2) If it was, inspect for a high water mark on the inside of the storage tub walls. If so, water damage may have occured.
3) Inspect the hydraulic fluid level line on the white, translucent reservoir that sits atop the pump.
4) Inspect the connectors and wiring harnesses.
Let us know how this shakes out,