The shop should have swapped the sensors for you, they're located in the valve stem as part of the assembly. Check your old wheels and tires for the sensors:
You may have to recalibrate the sensors, I've never done this on a gmt800 but the new bodies are the same way and should be similar
Resetting the TPMS Identification Codes
Each TPMS sensor has a unique identification
code. Any time you rotate your vehicle’s tires or
replace one or more of the TPMS sensors,
the identification codes will need to be matched to
the new tire/wheel position. The sensors are
matched to the tire/wheel positions in the following
order: driver’s side front tire, passengers side
front tire, passengers side rear tire, and driver’s
side rear tire using a TPMS diagnostic tool.
See your GM dealer for service.
The TPMS sensors may also be matched to each
tire/wheel position by increasing or decreasing
the tire’s air pressure. If increasing the tire’s
air pressure, do not exceed the maximum inflation
pressure indicated on the tire’s sidewall.
You will have two minutes to match the first
tire/wheel position, and five minutes overall to
match all four tire/wheel positions. If it takes longer
than two minutes, to match the first tire and
wheel, or more than five minutes to match all four
tire and wheel positions the matching process
stops and you will need to start over.
The TPMS sensor matching process is outlined
below:
1. Set the parking brake.
2. Turn the ignition switch to RUN with the
engine off.
3. Using the DIC, press the vehicle information
button until the RELEARN TIRE POSITIONS
message displays. If your vehicle does not
have the DIC buttons, press the trip odometer
reset stem located on the instrument panel
cluster until the RELEARN TIRE POSITIONS
message displays.
4. If your vehicle has the DIC buttons, press the
set/reset button. The horn will sound twice to
indicate the TPMS receiver is ready, and
the TIRE LEARNING ACTIVE message will
display. If your vehicle does not have the DIC
buttons, press and hold the trip odometer
reset stem until the horn chirps twice and the
TIRE LEARNING ACTIVE message is
displayed.
5. Start with the driver’s side front tire.
6. Remove the valve cap from the valve cap
stem. Activate the TPMS sensor by
increasing or decreasing the tire’s air pressure
for five seconds, or until a horn chirp
sounds. The horn chirp, which make take up
to 30 seconds to sound, confirms that the
sensor identification code has been matched
to this tire and wheel position. To decrease
air-pressure out of a tire you can use the
pointed end of the valve cap, a pencil-style air
pressure gage, or a key.
7. Proceed to the passenger’s side front tire, and
repeat the procedure in Step 6.
8. Proceed to the passenger’s side rear tire, and
repeat the procedure in Step 6.
9. Proceed to the driver’s side rear tire, and
repeat the procedure in Step 6.
10. After hearing the confirming horn chirp for the
driver’s side rear tire, the horn chirp will sound
two more times to signal the tire learning
mode is no longer active. Turn the ignition
switch to LOCK.
11. Set all four tires to the recommended air
pressure level as indicated on the Tire
and Loading Information label.
12. Put the valve caps back on the valve stems
Oh and :ttiwwop:
